Composite materials containing organic polymer-encapsulated metal organic frameworks

ABSTRACT

Metal organic resins, composite materials composed of the metal organic resins, and anion exchange columns packed with the composite materials are provided. Also provided are methods of using the composite materials to remove metal anions from a sample, methods of using the metal organic resins as fluorescence sensors for detecting metal anions in a sample, and methods of making the metal organic resins and the composite materials. The metal organic resins are amine-functionalized metal organic frameworks and their associated counter anions. The composite materials are composed of metal organic resin particles coated with organic polymers, such as alginic acid polymers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage Entry of InternationalApplication No. PCT/US2016/061241 that was filed Nov. 10, 2016, theentire contents of which are hereby incorporated by reference, whichclaims priority to U.S. provisional patent application No. 62/253,425that was filed Nov. 10, 2015, the entire contents of which are herebyincorporated by reference.

BACKGROUND

The contamination of water resources from toxic species represents amajor cause of global concern. Among the most commonly found pollutantsare various oxo-hydroxy anions. A characteristic example is Cr(VI)-oxospecies, found as dichromate (Cr₂O₇ ²⁻), hydrogen chromate (HCrO₄ ⁻) orchromate (CrO₄ ²⁻) ions, depending on the acidity/basicity of water.Cr(VI) is a well-known carcinogen that is released to the environmentfrom the leather tanning, cement, electroplating, and dyes industries,among others. Commonly used and inexpensive methods such asprecipitation of the ions from the solutions are not usually effectivein lowering the concentration of Cr(VI) below the acceptable limits.They also generate large amounts of metal-containing sludge. Reductionof Cr(VI) to Cr(III) with photocatalytic or other methods results in ahigh concentration of Cr(III) ions in the solution, and thus a secondarytreatment of the solution with a suitable sorbent is required to removethe generated Cr³⁺ ions. Adsorption and ion exchange are consideredhighly effective and relatively inexpensive methods for the treatment ofCr(VI)-containing waste. Several sorbents have been tested for thispurpose. Organic resins containing functional groups suitable forbinding of specific cations or anions are the most widely used sorbentsin various remediation processes and in the purification of drinkingwater. Commercially available resins with amine-functional groups haveshown promising Cr(VI) sorption properties. Such materials, however, arenot only relatively costly, but they also absorb Cr(VI) throughreduction-precipitation of Cr(III) species, which is not strongly boundto resins and can be leached from these sorbents under mild acidicconditions. Thus, such resins are actually oxidized-decomposed by Cr(VI)and cannot be regenerated and reused. Furthermore, these Cr(III)-loadedresins are not safe for disposal as non-hazardous solid waste due tosignificant Cr(III) leaching. On the other hand, layered doublehydroxides (LDHs), the typical inorganic anionic exchangers, areinexpensive, but show relatively slow sorption kinetics for Cr(VI) aswell as limited selectivity for it in the presence of competitive ions.

Metal Organic Frameworks (MOFs) decorated with organic groups havingstrong binding affinity for toxic ions may be ideal sorbents for variousremediation processes. Such functionalized materials, which can beprepared with facile methods and on a large scale, can be called MetalOrganic Resins (MORs), since they combine the organic functionalities ofthe amorphous organic resins (strong binding groups) and the orderedporous structure of crystalline MOFs. Thus, MORs with a combination offunctional-group based selectivity and a highly porous structure with anarrow pore size distribution favoring excellent sorption kinetics areexpected to present unprecedented efficiencies in practical separationprocesses. So far, there are very few reports on MORs showing sorptioncapability for Cr(VI). These MORs are of limited porosity and, as aresult, exhibit relatively slow sorption kinetics (ion-exchangeequilibrium is reached after several hours). Furthermore, most of themdisplay limited selectivity for Cr(VI) in the presence of excess ofvarious competitive anions and no regeneration/reuse capability. Inaddition, all reported materials have been tested with batch methods andno studies on their use in columns have been carried out. Note thatindustrial wastewater processes require continuous bed flow ion exchangecolumns. A sorbent material, in order to be appropriate for use incolumns, should display: a) high selectivity and fast sorption kineticsfor the targeted toxic ion; b) particle size suitable to allowcontinuous flow of wastewater through the column; and c) good mechanicalstrength to withstand high water pressures. MORs and other porousmaterials are usually characterized by very small particle size andinsufficient mechanical strength, which hinder their use in columns.Therefore, as-prepared MORs are not suitable for practical environmentalremediation applications.

SUMMARY

Metal organic resins, composite materials composed of the metal organicresins, and anion exchange columns packed with the composite materialsare provided. Also provided are methods of using the composite materialsto remove metal anions from a sample, methods of using the metal organicresins as photoluminescence sensor for detecting metal anions in asample, and methods of making the composite materials.

Metal organic resins, composite materials composed of the metal organicresins, and anion exchange columns packed with the composite materialsare provided. Also provided are methods of using the composite materialsto remove metal anions from a sample, methods of using the metal organicresins as fluorescence sensor for detecting metal anions in a sample,and methods of making the composite materials.

One embodiment of a composite material comprises: metal organic resinparticles comprising metal organic frameworks and associated counteranions, wherein the metal organic frameworks comprise metal nodescoordinated via organic molecular linkers to form a connected porousnetwork and further wherein the organic molecular linkers are protonatedand amine-functionalized; and an organic polymer coating the metalorganic resin particles.

One embodiment of an anion exchange column comprises: a column; and amixture of an inert granular material and a composite material, asdescribed herein, packed within the column.

One embodiment of a method of removing metal anions from a samplecomprises: exposing a sample comprising metal anions to the compositematerial, as described herein, whereby the metal anions undergo anionexchange with the counter anions of the composite material; andseparating the composite material from the sample.

One embodiment of a method of making a metal organic framework comprisesheating a mixture of a zirconium halide salt and NH₂—H₂BDC in an acidicaqueous solution, whereby a reflux reaction between the zirconium halidesalt and the NH₂—H₂BDC forms particles of a metal organic resincomprising zirconium nodes coordinated via organic molecular linkers ina connected porous network, wherein the organic molecular linkers areprotonated and amine-functionalized.

One embodiment of a method of making a composite material comprisesheating a mixture of a zirconium halide salt and NH₂—H₂BDC in an acidicaqueous solution, where NH₂—H₂BDC is 2-amino-terephthalic acid, wherebya reflux reaction between the zirconium halide salt and the NH₂—H₂BDCforms a suspension of metal organic resin particles in the solution, themetal organic resin comprising zirconium nodes coordinated via organicmolecular linkers in a connected porous network, wherein the organicmolecular linkers are protonated and amine-functionalized; and adding analkali metal alginate salt to the suspension, whereby the alkali metalalginate converts into alginic acid, which forms a water insolublealginic acid polymer coating on, and flocculates, the metal organicresin particles.

Another embodiment of a method of making a composite material comprisesforming an aqueous solution comprising an alkali metal alginate salt andmetal organic resin particles, the metal organic resin particlescomprising metal nodes coordinated via organic molecular linkers in aconnected porous network, wherein the organic molecular linkers areprotonated and amine-functionalized, whereby one or more monolayers ofalginate-saturated water form a coating on the metal organic resinparticles; adding an alkali earth metal halide salt to the aqueoussolution, whereby a water-insoluble coating of an alkali earth metalalginate forms around the metal organic resin particles; removing thecoated metal organic resin particles from the aqueous solution; andreacting the coated metal organic resin particles with a hydrogen halideto protonate the amine-functionalized metal organic frameworks andconvert the alkali earth metal alginate coating into an alginic acidpolymer.

Another embodiment of a method of making a composite material comprisesforming an aqueous solution comprising and alkali metal alginate saltand metal organic resin particles, the metal organic resin particlescomprising metal nodes coordinated via organic molecular linkers in aconnected porous network, wherein the organic molecular linkers areprotonated and amine-functionalized, whereby one or more monolayers ofalginate-saturated water form a coating on the metal organic resinparticles; and adding a hydrogen halide to the solution, whereby thehydrogen halide reacts with the alginate and the metal organic resinparticles to protonate the amine-functionalized metal organic frameworksand to form an alginic acid polymer coating around the organic resinparticles.

One embodiment of a metal organic resin comprises a metal organicframework and associated counter ions, the metal organic frameworkcomprising metal nodes coordinated via organic molecular linkers to forma connected porous network, wherein the organic molecular linkers areprotonated and amine-functionalized, the metal organic resin having theformula: [Zr₆O₄(OH)₈(H₂O)₄(H₂PATP)₄]X⁻ ₆, or the same formula, but withoxo ligands, aquo ligands, or a combination thereof in place of some orall of the hydroxo ligands, where H₂PATP is2-((pyridine-1-ium-2-ylmethyl)ammonio)terephthalate and X is amonovalent anion.

One embodiment of a method of detecting metal ions in a sample comprisescontacting a sample comprising the metal ions with a metal organic resincomprising metal organic frameworks and associated counter anions,wherein the metal organic frameworks comprise metal nodes coordinatedvia organic molecular linkers to form a connected porous network and theorganic molecular linkers are protonated and amine-functionalized;illuminating the sample with ultraviolet radiation, whereby the metalorganic resin absorbs the ultraviolet radiation and producesfluorescence emission; and measuring the fluorescence emissionintensity, the fluorescence emission profile, or both.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings.

FIG. 1 is a representation of the structure of (protonated) MOR-1 (seeExample 1) shown as a tetrahedral cage (based on the structure a UiO-66metal organic framework).

FIG. 2 is a schematic illustration for one method of forming andisolating a MOR-1-HA composite.

FIG. 3A depicts powder x-ray diffraction (PXRD) patterns for MOR-1 (asprepared), MOR-1-HA, MOR-1-HA@Cr(VI), and regenerated MOR-1-HA. FIG. 3Bshows nitrogen adsorption-desorption isotherms for MOR-1-HA andMOR-1-HA@Cr(VI) at 77 K.

FIG. 3C shows infrared (IR) spectra of MOR-1-HA and MOR-1-HA@Cr(VI).FIG. 3D shows high resolution Cr2p_(1/2) and Cr2p_(3/2) core-levelphotoelectron spectra (XPS) for MOR-1-HA de-convoluted into twocomponents. The minor signals with binding energy at 586.3 and 577.0 eVare attributed to Cr(III) traces resulted from the known reductioneffects under x-ray irradiation.

FIG. 4A shows equilibrium Cr₂O₇ ²⁻ sorption data for MOR-1-HA andUiO-66-HA materials. Solid lines represent the fitting of the data forMOR-1-HA and UiO-66-HA materials with the Langmuir andLangmuir-Freundlich models, respectively. FIG. 4B shows K_(d) values forMOR-1-HA and UiO-66-HA for initial Cr(VI) concentrations of 10.4 and 26ppm. FIG. 4C depicts the kinetics for Cr(VI) sorption by MOR-1-HA(initial Cr(VI) concentration=10.4 ppm, pH˜3). Inset graph: Fitting ofthe kinetics data with the first order Lagergren equationq_(t)=q_(e)[1−exp(−K_(L)t)], where q_(e)=the amount (mg/g) of metal ionabsorbed in equilibrium, K_(L)=the Lagergren or first-rate orderconstant (Fitting parameters: q_(e)=21.1(4) mg/g, K_(L)=3.3(1) min⁻¹).FIG. 4D shows % Total Cr removal by MOR-1-HA vs. pH (initial Cr(VI)concentration=10.4 ppm). For pH≥9, dissolution of the composite wasobserved.

FIG. 5A shows breakthrough curves for three column ion exchange runs(C=concentration of the effluent, C₀=initial Cr₂O₇ ²⁻ concentration ˜7ppm, pH˜3, flow rate 1 mL/min, one bed volume=3.5 mL, stationary phaseMOR-1-A/sand=0.05:5 g). FIG. 5B shows total Cr concentration in samples(bed volumes, each bed volume=3.5 mL) of natural spring water, to whichtrace Cr(VI) was added (initial total Cr concentration ˜450 ppb, pHadjusted to 3), after passing them through an ion-exchange column ofMOR-1-A/sand (0.05:5 g).

FIG. 6 shows total Se concentration in samples (bed volumes, each bedvolume=3.5 mL) of water solutions (initial SeO₃ ²⁻ concentration ˜736ppb, pH adjusted to 3), after passing them through an ion-exchangecolumn of MOR-1-A/sand (0.05:5 g).

FIG. 7 shows equilibrium SeO₄ ²⁻ sorption data for MOR-1-HA at pH˜7.7and 2.8. Solid lines represent the fitting of the data for with theLangmuir and Langmuir-Freundlich models respectively.

FIG. 8A depicts % selenate removal vs. initial selenate concentration atpH 7.7. FIG. 8B shows % selenate removal vs. initial selenateconcentration at pH 2.8. FIG. 8C depicts K_(d) for selenate removal vs.initial selenate concentration at pH 7.7. FIG. 8D shows K_(d) forselenate removal vs. initial selenate concentration at pH 2.8.

FIG. 9. K_(d) values for MOR-1-HA, MOR-1 (protonated), MOR-1(non-protonated) and UiO-66-HA (initial dichromate concentration=21.6ppm).

FIG. 10. Top: Equilibrium geometry of the (Ar—NH₂)CrO₃ complex inaqueous solution, along with selected structural parameters (bondlengths in Å and bond angles in degrees and the natural atomic chargeson the donor atoms constituting the coordination sphere and the centralCr atom. Bottom: Suggested mechanism for the Cr(VI) sorption anddesorption.

FIG. 11 depicts the structure of (protonated) MOR-1, represented as atetrahedral cage.

FIG. 12 shows schematics for the isolation of the MOR-1-HA compositewith the reflux-SA addition method.

FIG. 13 contains FE-SEM images for MOR-1 and MOR-1-HA and enlarged viewsof some particles of these materials.

FIG. 14A shows PXRD patterns of MOR-1, MOR-1-HA, MOR-1-HA@Cr(VI) and thecalculated pattern for UiO-66. FIG. 14B shows nitrogen sorptionisotherms at 77 K for MOR-1, MOR-1-HA and MOR-1-HA@Cr(VI).

FIG. 15 is an FE-SEM image for MOR-1-HA@Cr(VI) and an enlarged view ofone MOR-1-HA@Cr(VI) particle.

FIG. 16 shows equilibrium Cr₂O₇ ²⁻ sorption data for MOR-1-HA material(pH˜3). The solid line represents the fitting of the data with theLangmuir model.

FIG. 17A shows UV-Vis data from the kinetic experiments (initialdichromate concentration=21.2 ppm, pH˜3). The Cr₂O₇ ²⁻ anions are notdetectable with UV-Vis after 3 min and thus, the total Cr concentrationsof the solutions were determined by ICP-MS. FIG. 17B shows % Total Crremoval by MOR-1-HA vs. time (min).

FIG. 18 shows % Total Cr removal by MOR-1-HA vs. pH. The total Crconcentration was determined by ICP-MS.

FIG. 19(A) depicts breakthrough curves for five column ion exchange runs(C=Cr₂O₇ ²⁻ concentration of the effluent, C₀=initial Cr₂O₇ ²⁻concentration=6.4 ppm, pH˜3, flow rate ˜1.2 mL/min, one bed volume=3.5mL, stationary phase MOR-1-HA/sand=0.05 g:5 g). The lines are only aguide for the eye. FIG. 19(B) depicts breakthrough capacities obtainedfrom the five column ion exchange runs.

FIG. 20 shows total Cr concentration vs. bed volume (1 bed volume=3.5mL) of a water solution (total volume=1.1 L, initial total Crconcentration=394 ppb, pH˜3), after passing it through the MOR-1-HA/sand(0.05 g:5 g) column.

FIG. 21A depicts breakthrough curves for five column ion exchange runswith the wastewater sample A (C=Cr₂O₇ ²⁻ concentration of the effluent,C₀=initial Cr₂O₇ ²⁻ concentration=53.7 ppm, pH˜3.5, flow rate ˜1.2mL/min, one bed volume=3.5 mL, stationary phase MOR-1-HA/sand=0.05 g:5g). The lines are only a guide for the eye. FIG. 21B depictsbreakthrough capacities obtained from the five column ion exchange runs.

FIG. 22A depicts breakthrough curves for five column ion exchange runswith the wastewater sample B (C=Cr₂O₇ ²⁻ concentration of the effluent,C₀=initial Cr₂O₇ ²⁻ concentration=108 ppm, pH˜3, flow rate ˜1.2 mL/min,stationary phase MOR-1-HA/sand=0.05 g:5 g). The lines are only a guidefor the eye. FIG. 22B shows the excellent fit (solid line, correlationcoefficient R²=99.9%) of one breakthrough curve (4rth column run) withthe Thomas equation.

FIG. 23. Representation of a structure of MOR-2 as an 8-connected net.

FIG. 24A depicts isotherm data for Cr₂O₇ ²⁻ exchange. FIG. 24B depictsisotherm data for CrO₄ ²⁻ exchange. The line represents the fitting ofthe data with the Langmuir model.

FIG. 25 depicts kinetics of dichromate and chromate sorption by MOR-2(initial dichromate and chromate concentrations were 21.6 and 11.6 ppmrespectively).

FIG. 26A depicts breakthrough curves for five column ion exchange runs(C=Cr₂O₇ ²⁻ concentration of the effluent, C₀=initial Cr₂O₇ ²⁻concentration=108 ppm, pH˜3, flow rate ˜1.75 mL/min, one bed volume=3.5mL, stationary phase MOR-2-HA/sand=0.05 g:5 g). The lines are only aguide for the eye. Inset: Breakthrough capacities obtained from the fivecolumn ion exchange runs. FIG. 26B depicts fitting of the columnsorption data with the Thomas equation (fitting results: q_(max)=100mg/g, k_(Th)=0.006 L mg⁻¹ min⁻¹, R²=0.985).

FIG. 27 shows total Cr content (ppm) vs. bed volume (1 bed vole=3.5 mL)of a dichromate aqueous solution (initial total concentration=0.48 ppm,pH˜3), after passing it through the MOR-2-HA/sand column (flow rate˜1.75 mL/min, stationary phase MOR-2-HA/sand=0.05 g:5 g).

FIG. 28 shows breakthrough curves for five column ion exchange runs(C=CrO₄ ²⁻ concentration of the effluent, C₀=initial CrO₄ ²⁻concentration=52 ppm, pH˜7, flow rate ˜1.0 mL/min, one bed volume=3.5mL, stationary phase MOR-2-HA/sand=0.1 g:5 g). The lines are only aguide for the eye. Inset: Breakthrough capacities obtained from the fivecolumn ion exchange runs.

FIG. 29 shows total Cr content (ppm) vs. bed volume (1 bed vole=3.5 mL)of chrome plating solution A (initial total concentration=0.48 ppm,pH˜3), after passing it through the MOR-2-HA/sand column (flow rate˜1.75 mL/min, stationary phase MOR-2-HA/sand=0.05 g:5 g).

FIG. 30A shows breakthrough curves for five column ion exchange runswith the chrome plating solution B (C=CrO₄ ²⁻ concentration of theeffluent, C₀=initial CrO₄ ²⁻ concentration=52 ppm, pH˜8, flow rate ˜1.0mL/min, one bed volume=3.5 mL, stationary phase MOR-2-HA/sand=0.1 g:5g). The lines are only a guide for the eye. Inset: Breakthroughcapacities obtained from the five column ion exchange runs. FIG. 30Bshows the predicted (by the Thomas model) and experimentally found totalcolumn sorption capacities.

FIG. 31 depicts possible associations of the functional groups of MOR-2with chromate and dichromate species optimized at the BP86/6-31G(d,p)level of theory (C, grey; H, white; N, blue; O, red).

FIG. 32 shows solid state absorption profiles of MOR-2 and its CrO₄ ²⁻and Cr₂O₇ ²⁻ adducts.

FIG. 33 depicts fluorescence titration (λ_(exc)=360 nm) of activatedMOR-2 suspended in water upon the gradual addition of a 10⁻⁴ M aqueoussolution of K₂Cr₂O₇ [corresponding to 10.4 ppm of Cr(VI)]. Theexperiment was carried out at pH 3. The numbers correspond to totaladded Cr(VI) in ppb. The inset shows a photograph of two samples ofMOR-2 suspended in water at pH 3 and illuminated with a standardlaboratory UV lamp (360 nm). The left sample is free of Cr(VI) while theleft contains 500 ppb Cr(VI).

FIG. 34A shows fluorescence titrations (λ_(exc)=360 nm) of activatedMOR-2 with 10 ppm M stock solution of a chromium plating waste sample,with MOR-2 suspended in distilled water. The experiment was carried outat pH 3, and the numbers represent total added Cr(VI) in ppb. FIG. 34Bshows fluorescence titrations (λ_(exc)=360 nm) of activated MOR-2 with10 ppm M stock solution of a chromium plating waste sample, with MOR-2suspended in potable water containing 10.5 ppm SO₄ ²⁻ and 7.1 ppm NO₃ ⁻.The experiment was carried out at pH 3, and the numbers represent totaladded Cr(VI) in ppb.

DETAILED DESCRIPTION

Metal organic resins, composite materials composed of the metal organicresins, and anion exchange columns packed with the composite materialsare provided. Also provided are methods of using the composite materialsto remove metal anions from a sample, methods of using the metal organicresins as fluorescence sensor for detecting metal anions in a sample,and methods of making the composite materials.

The metal organic resin (MORs) comprise protonated, amine-functionalizedmetal organic frameworks with associated counter anions, such as halideanions. In some embodiments of the MORs the halide counter ions arechloride ions.

Metal-organic frameworks (MOFs) are a class of hybrid materialscomprising inorganic nodes and organic linkers. More specifically, theMOFs have a structure comprising inorganic (e.g., metal) nodes, alsoreferred to as centers, coordinated via organic molecular linkers toform a highly connected porous network.

In some embodiments the MORs are Zr⁴⁺ MORs of the UiO-66 family,comprising hexa-Zr^(IV) (Zr₆) nodes with tetratopic linkers. The metalorganic frameworks include those having Zr₆O₄(OH)₄ nodes and protonated,amine-functionalized 1,4-benzenedicarboxylate (BDC) linkers. Such MORscan be represented by the formula: [Zr₆O₄(OH)₄(NH₃ ⁺-BDC)₆]X⁻ ₆, where Xis a monovalent anion, such as a halide ion and are referred to hereinas MOR-1. The structure of these MORs is shown in FIG. 1, where thecounter anion is Cl⁻. Alternative formulations of the structure compriseoxo and/or aquo ligands in place of some or all of the hydroxo ligands.Other metal organic frameworks that can be protonated to provideprotonated, amine-functionalized metal organic frameworks include[Ti₈O₈(OH)₄(H₂N-BDC)₆], [Al₄(OH)₂(OCH₃)₄(H₂N-BDC)₃],[Al₃OCl(H₂O)₂(H₂N-BDC)₃], [AlOH(H₂N-BDC)].

The metal organic frameworks also include those having Zr₆O₄(OH)₈(H₂O)₄nodes and 2-((pyridin-1-ium-2-ylmethyl)ammonio)terephthalate (H₂PATP)linkers. Such MORs can be represented by the formula:[Zr₆O₄(OH)₈(H₂O)₄(H₂PATP)₄]X⁻ ₆, where X is a monovalent anion, such asa halide ion, and are referred to herein as MOR-2. The structure ofthese MORs is shown in FIG. 23, where the counter anion is Cl⁻.Alternative formulations of the structure comprise oxo and/or aquoligands in place of some or all of the hydroxo ligands.

The counter anions of the MORs are able to undergo anion exchange withvarious metal ions, including heavy metal ions, and, therefore, thecomposite materials have applications in metal anion remediation. (Asused herein the term “metal ion” refers to ions composed only of metalelements and anions that include metal elements and other elements, suchas oxygen.) The organic polymer that coats the MOR particles providesthe composite materials with increased mechanical strength and increasesthe average particle size and density of the material (relative to amaterial comprising the MOR particles without the encapsulatingcoating), making the coated MOR particles less prone to form a finepowder in aqueous solution. As a result, particles of the compositematerials can be formed with particle sizes and mechanical strengthsthat are able to withstand high water pressures and allow for acontinuous flow of a liquid sample through an anion exchange columncontaining the particles of composite materials. Alginic acid polymersare examples of organic polymers that can be used to encapsulate the MORparticles. Other examples include polyvinyl alkyl resins, such aspolyvinyl butyl resins, and polyalkylacrylates andpolyalkylmethacrylates, such as polymethylmethacrylates.

The coating on the MOR particles can be thin, so that it does notsubstantially affect the sorption capacity of the coated MOR particlesrelative to the uncoated MOR particles. By way of illustration, someembodiment of the polymer coated MOR particles, the organic polymercoating makes up no greater than 5 weight percent of the coated MORparticles. This includes embodiments of the polymer coated MOR particlesthat comprise no greater than 3 weight percent polymer coating andfurther includes embodiments of the polymer coated MOR particles thatcomprise no greater than 2 weight percent polymer coating.

Chromide ions and selenide ions, and other anions (e.g., oxide anions)containing chromium and selenium, are examples of ions that can beremoved from a sample via anion exchange using the composite materials.The MORs are able to remove dichromate and chromate ions. The removal ofthese ions is advantageous because they are toxic. As illustrated in theExamples, both the composite materials and the MORs from which thecomposite material are made have high sorption capacities for dichromateand/or chromate ions and are able to absorb these species rapidly. Forexample, sorption capacities of at least 250 mg/g, including at least300 mg/g can be achieved for dichromate ions and sorption capacities ofat least 200 mg/g, including at least 240 mg/g, can be achieved forchromate ions. (Methods for measuring the sorption capacity aredescribed in the Examples.) Other anions that can be removed from asample using the composite materials include precious metal-containinganions, such as precious metal-containing halide ions, such as PtCl₄ ²⁻and PdCl₄ ²⁻. MnO₄ ⁻ anions, ClO₄ ⁻ anions, and radioactive anions suchas TcO₄ ⁻ and ReO₄ ⁻ can also be selectively removed from a sample usingthe composite materials. Pentavalent arsenic (As(V)) and trivalentarsenic (As(III)) anions can also be removed. Some embodiments of theMORs, including those represented by the formula provided above, have ahigh selectivity for undergoing anion exchange with chromium andselenium ions, relative to other anions, such as Cl⁻, NO₃ ⁻, Br⁻ and SO₄⁻. Therefore, chromium and selenium ions can be effectively remediatedfrom a sample comprising one or more of those other anions, even whenthese other anions are present in excess.

The composite materials can be used to remove anion-exchangeable metalanions (targeted anions) from a sample by contacting the compositematerials with a sample, such as an aqueous solution, comprising thetargeted metal anions under conditions and for a time sufficient toallow the targeted metal anions to undergo anion exchange with thecounter anions of the MORs, and then separating the sample from thecomposite materials.

Because the composite materials are mechanically robust and can beformed as particles with tailored sizes and size distributions, they arewell suited for use as packing materials for ion exchange columns. Abasic embodiment of an anion exchange column comprises a columncharacterized by a length, an input opening for introducing a sampleinto the column at one end, and an output opening for releasing a samplefrom the column at another end. The column is packed with a packingmaterial that includes the composite material as an anion-exchangematerial. The packing material may consist of, or consist essentiallyof, particles of the composite material. However, the particles ofcomposite material may also be mixed with particles of another material,such as sand or other inert granular materials.

The composite materials are able to remove metal anions from sampleshaving a wide range of pH values, including pH values in the range from1-8. This is significant because many industries, such as the miningindustry, produce waste water samples that are highly acidic—having pHvalues of less than 7. For example, the composite materials can be usedto remediate samples having pH values in the range from 1 to 4.

Even under highly acidic conditions the composite materials are veryeffective at removing metal anions, such as chromium and selenium anionsfrom a sample. Under less acidic conditions, the effectiveness of thecomposite materials is even greater. By way of illustration, in someembodiments of the method of removing targeted metal anions (forexample, Cr- of Se-containing anions) from a sample, at least 80% of thetargeted metal anions are removed. This includes embodiments in which atleast 90%, at least 95%, and at least 98% (for example 95 to 99%) of thetargeted metal ions are removed. Thus, the composite materials are ableto remove metal ions to levels below those required by environmentalregulations in the United States and Europe. In addition, the materialsare inexpensive to synthesize and can be regenerated after undergoingremediative anion-exchange, allowing the anion-exchange columns to bereused multiple times with only a small—if any—loss in capacity.Industrial wastewaters that can be remediated using the compositematerials include those generated by the leather tanning, cement,electroplating, nuclear power, petroleum refining, mining, and dyesindustries. In addition, the composite materials can be used toremediate agricultural wastewater (e.g., farm run-off), contaminateddrinking water, or contaminated water from natural bodies of water.

Because the metal organic resins are so effective at removing targetions, only a small quantity of the composite material need be includedin an anion exchange column. Thus, in some embodiments of the anionexchange columns, the packing material in the column comprises no morethan 5 percent of the composite material by weight (wt. %), with theremainder comprising another material—typically an inert granularmaterial, such as sand. This includes embodiments in which the packingmaterial in the column comprises no more than 3 wt. % composite materialand further includes embodiments in which the packing material in thecolumn comprises no more than 1 wt. % composite material.

The composite materials can be made by different methods, as illustratedin the Examples. One method uses a reflux reaction to provide a rapid,high yield, relatively inexpensive, and environmentally friendlysynthesis. In this method, a metal halide salt and 2 amino-terephthalicacid (NH₂—H₂BDC) are refluxed in an acidic aqueous solution at anelevated temperature (that is, a temperature above room temperature) toinduce a reflux reaction between the halide salt and the 2amino-terephthalic acid to form a fine particulate suspension of anamine functionalized metal organic resin. By way of illustration, inorder to make MOR-1, or an MOR having the same formula as MOR-1, butwith oxo ligands, aquo ligands, or a combination thereof in place ofsome or all of the hydroxo ligands, ZrCl₄ salt can be used in the refluxsynthesis. Using this procedure, a high yield (e.g., 70% yield) of themetal organic resin can be obtained very quickly; with the maximum yieldof the metal organic resin being achieved in an hour or less.

Once the metal organic resin particle suspension has been formed, analkali-metal alginate salt, such as sodium alginate, is added to thesuspension where it is converted into alginic acid. The alginic acidforms a water insoluble alginic acid polymer coating on the metalorganic resin particles, which then flocculate in the solution. Thesepolymer coated organic resin particles can then be precipitated from thesolution and isolated using, for example, filtration or centrifugation.The resulting particulate material can then be treated with an acid,such as a strong inorganic acid (e.g., HCl) to dissolve the remainingNH₂—H₂BDC ligands and to protonate the amino functional groups of thematerial. An example of this reflux synthesis-based process isillustrated in detail in Example 2.

In another method of making the composite materials, an aqueous solutionof an alkali metal alginate salt and the metal organic resin particlesis formed. In solution, one or more monolayers of alginate-saturatedwater form a coating on the metal organic resin particles. An alkaliearth metal halide salt is then added to the aqueous solution, whereby awater-insoluble coating of an alkali earth metal alginate forms aroundthe metal organic resin particles. These particles can then be removedfrom the aqueous solution and reacted with a hydrogen halide toprotonate the amine-functionalized metal organic frameworks and convertthe alkali earth metal alginate coating into an alginic acid polymercoating. An example of this synthesis process is illustrated in detailin Example 1.

In yet another method of making a composite material, an aqueoussolution of an alkali metal alginate salt and the metal organic resinparticles is formed. In solution, one or more monolayers ofalginate-saturated water form a coating on the metal organic resinparticles. A hydrogen halide is then added to the solution, whereby thehydrogen halide reacts with the alginate and the metal organic resinparticles to protonate the amine-functionalized metal organic frameworksand to form an alginic acid polymer coating around the organic resinparticles. An example of this synthesis process is illustrated in detailin Example 1.

The polymer coated metal organic resin particles formed by these methodstypically have a mean particle size in the range from about 100 nm toabout 500 nm, including from about 100 nm to about 300 nm. The particlesare microporous, typically having mean pore sizes in the range fromabout 3 Å to about 12 Å. Although mean particle and pore sizes outsideof these ranges can be also be achieved.

Another aspect of this invention is a method for making the metalorganic resin designated above as MOR-2 and the use of this, and otheramine-functionalized metal organic resins, in fluorescence-based sensorsfor the detection of metal ions. As illustrated in Example 3, MOR-2 canbe synthesized via a solvothermal reaction of a zinc halide salt with2-((pyridine-2-ylmethyl)amino)terephthalic acid (H₂PATP) ligand.Different metal halide salts can be used in the solvothermal reaction toform a MOR having the structure of MOR-2, but with different metals,such as Hf⁴⁺, at the metal nodes. The metal organic resin can be treatedwith a halide acid, such as HCl, to protonate its pyridine and aminemoieties, thereby providing pyridinium and ammonium functional groupscharge balanced by halide anions.

Embodiments of the metal organic resins, including MOR-1, MOR-2 andvariations of these in which oxo ligands, aquo ligands, or a combinationthereof replace some or all of the hydroxo ligands, have absorptionbands in the ultraviolet region of the electromagnetic spectrum andproduce fluorescence emission upon irradiation with ultraviolet light.This fluorescence emission is quenched by the sorption of metal anionsby the metal organic resin. Thus, the photophysical properties of themetal organic resins can be harnessed to detect metal ions, includingheavy metal ions, in a sample.

In a method of detecting metal ions in a sample, the metal organic resinis contacted with a sample comprising the metal ions and metal ions inthe sample undergo anion exchange with the counter anions of the metalorganic resin. The metal organic resin is then irradiated withultraviolet radiation and the resulting fluorescence spectrum ismeasured. Because the intensity of the fluorescence emission decreasesand/or the fluorescence emissions peaks shift with increasing absorbedmetal ion concentration, the intensity and/or fluorescence emissionprofile of the measured fluorescence can be compared to the fluorescenceintensity and fluorescence emission profile of a standard sample thatincludes the metal organic resin without the metal ions to quantify theconcentration of metal ions in the sample. The metal organic resin canbe contacted with the sample by, for example, adding it into the sampleor by passing the sample over the metal organic resin. The latterprocess can be achieve by providing polymer coated metal organic resinparticles, as described in detail herein, in an anion exchange column.Example 3 illustrates the use of an MOR-2 based fluorescence sensor fordetecting both chromate and dichromate metal ions in solution. Othermetal anions that can be detected with the fluorescence emission sensorsbase on this and other MORs described herein include, but are notlimited to, [PdCl₄]²⁻ and [PdCl₄]²⁻.

Example 1

This example illustrates an anion exchange composite material based on aprotonated amine-functionalized metal organic framework, called MetalOrganic Resin-1 (MOR-1), and alginic acid (HA). Additional detailsregarding the composite material and its use in the capture ofhexavalent chromium can be found in Rapti et al., Chem. Sci., 2016, 7,2427-2436 and its Supplementary Information, the entire contents ofwhich is incorporated herein by reference.

The composite material can be synthesized via a simple and inexpensivemethod. The sorbent shows an exceptional capability to rapidly andselectively sorb Cr(VI) under a variety of conditions and in thepresence of several competitive ions. The composite sorbent can besuccessfully utilized in an ion-exchange column. Remarkably, an ionexchange column with only 1% wt. MOR-1-HA and 99% wt. sand (an inert andinexpensive material) is capable of reducing moderate and trace Cr(VI)concentrations well below the acceptable limits for water (effluent Crconcentrations ≤1 ppb). Additionally, this column is highly efficient inremoving Se (in the form of SeO₃ ²⁻ and SeO₄ ²⁻), with the effluent Seconcentrations being ≤1 ppb (˜50 times smaller than the USAEnvironmental Protection Agency (EPA)-defined limit for Se).

The anion exchange composite material is based on the [Zr₆O₄(OH)₄(NH₃⁺-BDC)₆]Cl₆ MOR (MOR-1) and alginic acid (HA) polymer(NH₂—H₂BDC=2-amino-terephthalic acid). The MOR is the analogue of theUiO-66 material containing NH₃₊ functional groups (FIG. 1). Throughdetailed batch studies, the highly efficient and selective anionexchange properties of the composite for Cr₂O₇ ²⁻ ions are revealed. Thesuccessful use of MORs, in the form of MOR-HA composite, in an ionexchange column are demonstrated. The stationary phase in this column isa mixture of MOR-1-HA composite and sand (an inert and inexpensivematerial). Remarkably, a column with MOR-1-HA/sand stationary phasecontaining only 1% w/w MOR-1-HA was found to be capable of reducingmoderate and trace levels of Cr(VI) well below the allowed safe levels(EU and USA-EPA limits for total Cr in water are 50 and 100 ppbrespectively), despite the presence of large excess of competitive ions.Furthermore, the column could be easily regenerated and reused severaltimes with almost no loss of its capacity. The efficiency and relativelylow cost of this ion exchange column makes it attractive for use in thedecontamination of wide variety of Cr(VI)-containing wastes.

Zr⁴⁺ MORs of the UiO-66 family are useful materials for sorptionapplications due to their high surface areas, easy incorporation offunctional groups and hydrolytic-thermal stability. However, asmentioned above, as-prepared MORs are fine powders and are not suitablefor practical ion exchange applications. This is particularly true forUiO-66 type MORs usually isolated as nanoparticles. In fact, as-preparedUiO-66 type MORs form fine suspensions in water and cannot be easilyseparated from it. The latter is a major drawback for the application ofsuch materials as stationary phases in columns. To this end, a modifiedalginate encapsulation method was applied to prepare UiO-66type-composite materials. This encapsulation method involves: a)addition of the sorbent to be encapsulated (i.e. the MOR) into a watersolution of sodium alginate (SA), so that one or more monolayers ofalginate-saturated water cover each particle of the sorbent (FIG. 2, toppanel); b) addition of CaCl₂ to the SA-sorbent suspension so that themonolayer is immediately converted to calcium alginate (CA), forming awater-insoluble polymer shell around the sorbent particulates (MOR-1-CA)(FIG. 2, middle panel); and c) treating the MOR-1-CA composite withhydrochloric acid to produce [Zr₆O₄(OH)₄(NH₃₊-BDC)₆]Cl₆-HA (MOR-1-HA)(HA=alginic acid) (FIG. 2, bottom panel). Note that only 4% wt. ofalginate (i.e., alginate:MOR-1 mass ratio used was ˜0.04) was sufficientfor the composite to be formed and thus, the MOR was not encapsulated bythick HA particles that would hinder the diffusion of ions into the MORpores. Alternatively, the MOR-1-HA composite could be prepared directlyby adding HCl into a suspension of MOR-1 in SA water solution. Thecomposite material could be also synthesized by heating a mixture ofZrCl₄+NH₂—H₂BDC in water-acetic acid solution under reflux conditionsfor a few hours, which results in the formation of a fine MORsuspension. Adding SA+HCl to the suspension produces the MOR-HAcomposite in high yield. This synthetic method is inexpensive andenvironmentally friendly, since no organic solvent is used (except for arelatively small quantity of the inexpensive acetic acid).

EDS data for the MOR-1-HA sample indicate a Zr:Cl ratio of ˜1, which isin agreement with the protonation of the six amino groups of the Zr₆cluster and the presence of six Cl⁻ counter ions. Thermogravimetricanalysis (TGA) was used to determine the lattice water molecules (21water molecules). Powder X-ray diffraction (PXRD) data indicated thatthe MOR retained its structure in the composite form (FIG. 3A). TheBrunauer-Emmett-Teller (BET) surface area of the MOR-1-HA was 1004 m²/g(FIG. 3B), a value within the range of surface areas found foramino-functionalized UiO-66 materials.

Detailed Cr(VI) sorption studies for MOR-1-HA were performed at low pH(pH˜3), in order to imitate the usual acidic conditions of Cr(VI)industrial waste (for example tannery wastewater). Under suchconditions, the predominant Cr(VI) species were Cr₂O₇ ²⁻ (with somecontribution from HCrO₄ ⁻ at dilute solutions). The ion exchange processcan be described with the following equation:

EDS data revealed no Cl⁻ anions in the Cr₂O₇ ²⁻-loaded material. ICP-MS,EDS and UV-Vis data (see below) indicated a Zr:Cr ratio of 0.9-1.2,close to the expected one (theoretical Zr:Cr=1, considering theinsertion of 3 Cr₂O₇ ²⁻ per Zr₆ cluster). PXRD data revealed that theMOR structure was retained after the incorporation of the dichromateanions (FIG. 3A). There was a drastic decrease, however, in the BETsurface area for the Cr₂O₇ ²⁻-loaded material, indicating the pores ofthe structure were filled by Cr₂O₇ ²⁻ ions. Specifically, after theinsertion of Cr₂O₇ ²⁻ anions, the surface area for MOR-1-HA dropped from˜1000 to 36 m²/g (FIG. 3B).

The presence of Cr(VI) species in the exchanged MOR-1-HA material wasfurther shown using infrared (IR) and X-ray photoelectron spectroscopy(XPS). The IR spectrum (FIG. 3C) of the exchanged material showed theexistence of a peak at ˜924 cm⁻¹ (not present in the spectra of pristineMOR1-HA material) assigned to the anti-symmetric CrO₃-stretch (for moredetailed interpretation of IR data see also below). XPS data revealedthe presence of Cr2p_(1/2) and Cr2p_(3/2) peaks, with their maincomponents corresponding to binding energies of 588.1 and 579.3 eV (FIG.3D). These binding energies are consistent with those of Cr(VI).

To gain further insight into the Cr₂O₇ ²⁻ sorption properties of theMOR-1-HA material, batch studies were first performed. By immersing theMOR-1-HA material in a Cr₂O₇ ²⁻ solution, the removal of Cr₂O₇ ²⁻ wasaccomplished very quickly (within a few minutes), something that couldbe visually observed by the decolorization of the solution and colorchange of the sorbent. The Cr₂O₇ ²⁻ ion exchange equilibrium data forMOR-1-HA composite are shown in FIG. 4A. The description of the data canbe provided by the Langmuir model. The sorption capacity for MOR-1-HAwas 242(17) mg Cr₂O₇ ²⁻/g of sorbent (or 242/0.96=252 mg/g of MOR-1),which exceeded those reported for other metal organic materials (60-100mg/g) and most of known inorganic and organic anion exchangers. Thissorption capacity was consistent with the absorption of ˜2.7(3) moles ofCr₂O₇ ²⁻ per formula unit of the MOR, which is close to the expectedmaximum sorption capacity of the material (3.0 mol per formula unit).The affinity of the MOR-1-HA for dichromate could be expressed in termsof the distribution coefficient K_(d), which is given by the equation

${K_{d} = \frac{V\left\lbrack {\left( {C_{0} - C_{f}} \right)/C_{f}} \right\rbrack}{m}},$where C₀ and C_(f) are the initial and equilibrium concentrations ofCr₂O₇ ²⁻ (ppm), V is the volume (ml) of the testing solution, and m isthe amount of the ion exchanger (g) used in the experiment. Values forK_(d) equal to 10⁴ L/g and above this value are considered excellent.The maximum K_(d) ^(Cr) ₂ ^(O) ₇ values for the MOR-1-HA material,obtained from the batch equilibrium studies, were in the range1.2-5.5×10⁴ L/g (FIG. 4B), which revealed the exceptional affinity ofthe materials for dichromate ions. It should be noted that MOR-1-HAsamples loaded with Cr₂O₇ ²⁻ could be easily regenerated by treatingthem with concentrated HCl solutions (1-4 M). The PXRD pattern of theregenerated MOF-1-HA was almost identical with that of the as preparedMOR-1-HA material. The regenerated MOR-1-HA showed similar dichromateexchange capacity (220-230 mg/g) to that of the pristine material (moredetailed regeneration studies were performed for the ion exchangecolumns, see below).

The kinetics of the Cr₂O₇ ²⁻ exchange of the MOR-1-HA composite werealso studied. The results indicated that the capture of Cr₂O₇ ²⁻ by thecomposite was remarkably fast (FIG. 4C). Within only 1 min ofsolution/composite contact, ˜94.2% of the initial Cr amount (C=10.4 ppm,pH˜3) was removed by the solution. After 3 min of solution/compositecontact, the Cr(VI) ion exchange almost reached its equilibrium with˜97.5% removal capacity. These kinetic data can be fitted with the firstorder Lagergren equation (FIG. 4C, inset). From these data, it is clearthat the ordered highly porous structure of MOR-1-HA facilitating thediffusion of ions in and out of pores, and the presence of protonatedamino-functional groups strongly interacting with the Cr₂O₇ ²⁻ anions,resulted in a sorbent with exceptionally rapid sorption kinetics.

Although the Cr(VI) ion exchange studies for MOR-1-HA were performed atpH˜3 in order to evaluate the capability of the sorbent to operate underacidic conditions usually present in industrial waste, the compositematerial was found to be capable of absorbing Cr(VI) from solutions of arelatively wide pH range (1-8), FIG. 4D. Specifically, it showed 91-98%total Cr removal capacities in pH˜3-8, whereas it retained high Crremoval capability, even under highly acidic conditions (80.5 and 90.2%removal capacities at pH˜1 and 2 respectively).

Cr₂O₇ ²⁻-bearing industrial effluent also was found to contain a numberof competitive anions, such as Cl⁻, NO₃ ⁻, Br⁻ and SO₄ ²⁻, in highconcentrations. Thus, competitive Cr₂O₇ ²⁻/Cl⁻, Cr₂O₇ ²⁻/Br⁻, Cr₂O₇²⁻/NO₃ ⁻ and Cr₂O₇ ²⁻/SO₄ ²⁻ sorption experiments for MOR-1-HA wereperformed. An exceptional ability of MOR-1-HA to absorb Cr₂O₇ ²⁻(initial concentration=54 ppm, pH˜3) almost quantitatively (81.6-97.6%dichromate removal capacity) and very high K_(d) ^(Cr) ₂ ^(O) ₇(4.4×10³-4×10⁴ L/g) in the presence of tremendous (up to 1000-fold)excess of Cl⁻, Br⁻, or NO₃ ⁻ was observed, which indicates very highselectivity of MOR-1-HA for Cr₂O₇ ²⁻ against these anions. SO₄ ²⁻ as abivalent anion is expected to be a stronger competitor than monovalentanions for dichromate anion exchange. Nevertheless, even with relativelylarge (20-80-fold) excess of SO₄ ²⁻, MOR-1-HA retained a very good Cr₂O₇²⁻ removal efficiency (40-68%) and relatively high K_(d) ^(Cr) ₂ ^(O) ₇values (up to 2.1×10³ mL/g).

Comparative Batch Ion-Exchange Studies

For comparison, batch Cr₂O₇ ²⁻ sorption studies (at pH˜3) were performedfor: a) protonated MOR-1 [Zr₆O₄(OH)₄(NH₃ ⁺-BDC)₆]Cl₆ (MOR-1 treated withHCl 4 M); b) non-protonated MOR-1 [Zr₆O₄(OH)₄(NH₂—BDC)₆] (preparedwithout adding acid in the reaction mixture); and c) UiO-66 MOF([Zr₆O₄(OH)₄(BDC)₆])-HA composite. The results indicated that thesorption capacities of protonated MOR-1 and non-protonated MOR-1 weresimilar to each other (247±10 and 267±23 mg/g respectively) and alsoclose to that of the MOR-1-HA composite, whereas the sorption capacity(129±18 mg/g) of UiO-66-HA was almost half of that for MOR-1-HA. Theefficiency, however, of protonated MOR-1 and MOR-1-HA for sorption ofdichromate in relatively low initial concentrations, as revealed by theK_(d) ^(Cr) ₂ ^(O) ₇ values, was significantly higher than that ofnon-protonated MOR-1 and UiO-66-HA. Specifically, UiO-66-HA andnon-protonated MOR-1 materials showed K_(d) ^(Cr) ₂ ^(O) ₇ values of 2.3and 6.5×10³ L/g, respectively, for initial dichromate concentration of˜21.6 ppm (FIG. 9), which were one order of magnitude less than those(˜5.5×10⁴ L/g) for MOR-1-HA and protonated MOR-1 samples (FIG. 9).

As the above results revealed, both protonated and non-protonated MOR-1materials displayed similar maximum sorption capacities, since at theacidic environment the NH₂-groups will be eventually protonated and theinserted Cl⁻ can be exchanged by dichromate anions. However, at lowinitial Cr(VI) concentrations, the materials pre-treated with acid(i.e., protonated MOR-1 and MOR-1-HA) were much more effective for thesorption of Cr(VI), as revealed by their much higher K_(d) valuescompared to that of MOR-1 used without any pre-treatment (non-protonatedMOR-1). Presumably, the pre-existence of exchangeable Cl⁻ anions in theprotonated materials enhances the kinetics of the Cr(VI) sorption,whereas the Cr(VI) sorption by the non-protonated MOR is a slowertwo-step process involving first protonation of theamine-sites/insertion of Cl— anions and then exchange of Cl⁻ by Cr(VI)species. The enhancement of sorption kinetics was particularly importantin the case of low initial Cr(VI) concentrations, which were not aseffective as the high Cr(VI) levels at shifting the ion-exchangeequilibrium towards the Cr(VI)-containing material. The aboveexplanation was supported by a kinetic study of the Cr₂O₇ ²⁻ exchange ofthe non-protonated MOR-1 using a relatively low initial dichromateconcentration (21.6 ppm, pH˜3). The results showed that after 1 min ofsolution/MOR contact only 24% Cr₂O₇ ²⁻ removal was achieved, whereaseven after 60 min of reaction significant amount of dichromate remainedin the solution (˜76% Cr₂O₇ ²⁻ removal). These data are in contrast withthe corresponding kinetic results for MOR-1-HA, which indicated almostquantitative sorption of dichromate anions within only 1 min ofsolution/composite contact (FIG. 7). Furthermore, fitting of the kineticdata for the non-protonated MOR-1 with the Lagergren's first orderequation revealed a rate constant of 0.55±0.14 min⁻¹, which is six-timessmaller than that for the Cr₂O₇ ²⁻ sorption by MOR-1-HA. Thisimprovement of kinetics via the protonation of the material is key forits substantially higher column sorption efficiency compared to that ofnon-protonated sorbent.

The next step in these investigations was the study of the column Cr(VI)sorption properties of MOR-1-HA material. At this point, it should bementioned that efforts to use as prepared MOR-1 (even after mixing itwith inert materials such as sand) in columns were unsuccessful, sinceMOR-1 forms fine suspensions in water that pass through the column.Thus, only MOR-1-HA composite could be successfully employed for columnsorption studies. The stationary phase in the columns was a mixture ofMOR-1-HA and sand, a common inexpensive and inert material typicallyused in columns. The use of such mixtures instead of the pure compositewas found to have several advantages: a) the pieces of the compositematerial were immobilized (not disturbed and moved by the water flow)and separated by particles of sand, thus ensuring a continuous waterflow through the column; b) the pressure exerted by water on thecomposite was reduced, since part of this pressure was absorbed by thesecond material (sand); and c) mixing the composite material with a verylow cost material such as sand would be economically attractive. Itshould be noted that no clogging of MOR-1-HA/sand columns was observedafter passing several litres of solutions through them. Remarkably, itwas found that stationary phases containing only ˜1% wt. of MOR-1-HA and99% wt. sand were very effective for the removal of either high or lowconcentration Cr(VI) from aqueous solutions of various compositions. Itcould be seen that highly concentrated dichromate solution (C˜1080 ppm,pH˜3) was decolorized after passing it through the MOR-1-HA/sand column.Also, the stationary phase changed color from cream white toorange(red)-brown after the sorption of significant amount of Cr₂O₇ ²⁻anions. The sorbent could be easily regenerated by washing it with ˜4 MHCl (FIG. 4B). The regeneration could be visually observed by therestoration of the cream white color of the initial MOR-1-HA/sandstationary phase. Detailed column sorption studies were performed withCr₂O₇ ²⁻ solutions of low and trace levels, which cannot be treated withcommon methods such as precipitation. Specifically, column sorption of asolution (pH˜3) of dichromate anions with concentration of 6 ppmresulted in almost no Cr(VI) species (removal capacities 298% and totalCr concentrations ≤47 ppb, i.e. below the EU and USA-EPA defined limitfor total Cr) for 80 bed volumes (Bed volume=[bed height (cm)cross-sectional area (cm²)] mL) of the effluent (FIG. 5A). Afterregeneration, a breakthrough curve almost identical to that of the firstrun was obtained, whereas only a small decrease (˜3 bed volumes) of thebreakthrough capacity was observed for a third run of the column (FIG.5A). Column sorption studies have been also conducted with dichromatesolutions (pH˜3, C=7 ppm) containing 100-fold excess of each of Cl⁻, Br⁻and NO₃. Still, the ion exchange column showed significant breakthroughcapacity (˜43 bed volumes), which was retained exactly the same afterits regeneration. Because of the excellent Cr₂O₇ ²⁻-column sorptionproperties described above, it was decided to examine the applicabilityof MOR-1-HA/sand column for remediation of real world water samplesintentionally contaminated by trace concentrations of Cr₂O₇ ²⁻.Specifically, the performance of this ion exchange column was tested forthe decontamination of natural spring water solutions (with the pH ofthe solution adjusted to 3), to which trace levels of Cr₂O₇ ²⁻ (total Crconcentration analyzed with ICP-MS ˜450 ppb) were added. Note that thetested water solutions contained 27, 28 and 305-fold excess of SO₄ ²⁻,NO₃ ⁻ and Cl⁻ anions, compared to the initial concentration ofdichromate anions. The results indicated that at least 21 samples (bedvolumes) collected after running the column three times (withregeneration of the column after each run) contained total Cr in aconcentration ≤1 ppb, i.e. well below the allowed total Cr concentrationin water, FIG. 5B. Finally, it should be mentioned that no Zr was foundin the effluent samples, thus excluding MOR leaching from the column.

The MOR-1-HA sorbent was also capable of absorbing Se species. Thus, theMOR-1-HA/sand column was very effective for the sorption of SeO₃ ²⁻ intrace levels. Specifically, after passing a solution of SeO₃ ²⁻ (initialconcentration=736 ppb, pH˜3) through the column, at least 7 samples (bedvolumes) contained Se in concentrations well below the USA-EPA limit (50ppb) for Se in water (FIG. 6).

In addition, batch SeO₄ ²⁻ sorption studies were performed. The selenateion-exchange equilibrium data at two different pH values are shown inFIG. 7. These data could be fitted with the Langmuir orLangmuir-Freundlich models. The maximum sorption capacity was found tobe 65(3) and 135(22) mg/g at pH˜7.7 and 2.8, respectively. These valuescorrespond to 1.1(1) (pH˜7.7) and 2.3(4) (pH˜2.8) moles of selenate performula of MOR-1. Thus, the selenate sorption capacity of the materialwas close to its theoretical maximum value (3 selenate moles per formulaof MOR-1) only at pH˜2.8. Presumably, all amino-groups remain protonatedat the low pH value and thus, the material contains more Cl⁻ anions(counter ions) to be exchanged by SeO₄ ²⁻.

The sorbent was particularly effective for the removal of selenate inmoderate and trace concentrations. It could be seen that the % SeO₄ ²⁻removal capacities were 90-98% for initial concentrations of 2.2-35.7ppm at pH˜7.7 (FIG. 8A), whereas the corresponding values at pH˜2.8 were98-99.9% (FIG. 8B). The maximum K_(d) values for selenate removal were5×10⁴ and 6.7×10⁵ mL/g at pH˜7.7 and 2.8, respectively. (FIGS. 8(C) and8(D) The above indicate significantly higher affinity of the sorbent forselenate removal at low pH. Thus, the sorbent may be particularlyeffective for removing Se species from mining wastewater, which isusually acidic.

Mechanism of Cr(VI)-Sorption

To provide an explanation for the remarkable selectivity of theprotonated amino functionalized material for dichromate anions, theinteraction energies of Cr₂O₇ ²⁻, HOCrO₃ ⁻, Cl⁻, Br⁻, NO₃ ⁻, HOSO₃ ⁻ andSO₄ ²⁻ anions with the [Zr₆O₄(OH)₄(NH₃ ⁺-BDC)₆]Cl₆-HA (MOR-1-HA) wascalculated, represented by the simple anilinium Ar—NH₃ ⁺ cation,employing DFT methods. The calculated interaction energies along withselected structural parameters of the respective associations arecompiled in Table 1.

TABLE 1 Interaction energies, IE (in kcal/mol) and selected structuralparameters (bond lengths in Å, bond angles in degrees) for the Ar—NH₃ ⁺ 

 A (A = Cl⁻, Br⁻, NO₃ ⁻, HOSO₃ ⁻, HOCrO₃ ⁻, Cr₂O₇ ²⁻) associations inaqueous solutions calculated by the wB97XD/Def2-TZVPPD/PCM computationalprotocol. Anion IE R(O 

 H—N) R(N—H) <O 

 H—N Cl⁻ 11.7 1.942 1.070 176.1 Br⁻ 9.9 2.139 1.061 176.0 NO₃ ⁻ 13.31.597 1.071 175.7 HOSO₃ ⁻ 12.0 1.660 1.055 166.6 2.478 1.020 110.3HOCrO₃ ⁻ 12.6 1.672 1.052 159.4 2.142 1.024 127.9 Cr₂O₇ ²⁻ 15.5 1.6371.055 163.1

Interestingly, the calculations indicated that SO₄ ²⁻ abstracts a NH₃ ⁺proton from the Ar—NH₃ ⁺ cation yielding HOSO₃ ⁻ anions via anexothermic process (exothermicity of −26.7 kcal/mol). Thus, in Cr₂O₇²⁼/SO₄ ²⁻ competition ion-exchange reactions with MOR-1-HA, the actualcompetitor for dichromate exchange was HOSO₃ ⁻. The latter as amonovalent anion is expected to be less competitive than SO₄ ²⁻ forCr(VI) sorption. This can be one of the reasons for the relatively highselectivity of MOR-1-HA for Cr(VI) vs. SO₄ ²⁻, which was experimentallyobserved.

Among the anions studied, the Cr₂O₇ ²⁻ anion shows the strongestinteractions (15.6 kcal/mol). However, the estimated values of theinteraction energies for the Ar—NH₃ ⁻. . . A (A=Cl⁻, Br⁻, NO₃ ⁻, HOSO₃⁻, HOCrO₃ ⁻, Cr₂O₇ ²⁻) associations could not fully explain the highselectivity of the material under study towards Cr₂O₇ ²⁻ anions and thelimited selectivity for the rest of the competitive anions in theseries. Therefore, this selectivity could be due to much strongerinteractions between the Cr₂O₇ ²⁻ anions and the Ar—NH₃ ⁺ cation.Experimental IR-data indicated that the amine-deformation band wassignificantly red-shifted for MOR-1-HA@Cr(VI) (1565 cm⁻¹) compared tothat for pristine MOR-1-HA (1590 cm⁻¹) and as prepared MOR-1 (1580 cm⁻¹)samples (FIG. 3C). Furthermore, the IR peak at 1620 cm⁻¹ (assigned toring stretching vibration) in the spectrum of MOR-1-HA@Cr(VI), which wasalso present in the IR spectrum of non-protonated MOR, but it is notshown or is of very weak intensity in the spectrum of MOR-1-HA, isindicative of NH₂ rather than NH₃ ⁺-containing phenyl ring. In addition,the solid-state UV-Vis spectrum for MOR-1-HA@Cr(VI) revealed a broadfeature (around 500 nm) in the visible region (not shown in the spectrumof as-prepared MOR-1 and MOR-1-HA), which may be due to charge transferfrom the electron rich NH₂—BDC²⁻ ligand to the Cr(VI) species (LMCT).The above support strong NH₂—Cr(VI) interactions in MOR-1-HA@Cr(VI).

This suggests the transformation of dichromate to Cr^(VI)O₃ species,which in turn forms the tetrahedral [(Ar—NH₂)CrO₃] complex. To test thishypothesis, the equilibrium geometry of the [(Ar—NH₂)CrO₃] complex inaqueous solution was optimized at the wB97XD/Def2-TZVPPD level oftheory, FIG. 10. The formation of such a complex may be promoted by thesignificant acidity of anilinium ion (the pKa of the anilinium ion islower than 4.6). Anilinium may interact with the bridging O atom(nucleophilic center) of Cr₂O₇ ²⁻, enforcing the rupture of a O—Crbridging bond with concomitant coordination of aniline to CrO₃ fragmentand formation of HOCrO₃ ⁻ anion. The latter subsequently re-equilibratesto produce Cr₂O₇ ²⁻:Ar—NH₃ ⁺+Cr₂O₇ ²⁻→(Ar—NH₂)CrO₃+HOCrO₃ ⁻  (4)2HOCrO₃ ⁻→Cr₂O₇ ²⁻+H₂O  (5)

The condensation of the HOCrO₃ ⁻ anion to form Cr₂O₇ ²⁻ is particularlyan enthalpy driven reaction with a dimerization constant K=159 atstandard conditions.

The formation of the (Ar—NH₂)CrO₃ complex is an almost thermoneutralprocess, the endothermicity found to be 1.8 kcal/mol. The presence ofsix NH₃ ⁺ functional groups per Zr₆ cluster affords six moles of the(Ar—NH₂)CrO₃ complex, thus accounting well for the experimentallyobserved sorption of ˜3 moles of Cr₂O₇ ²⁻ per formula unit of the MOR(FIG. 10).

Oxochromium(VI)-amine complexes are well-known compounds and many ofthem have been used as oxidants in organic synthesis. The brick-redcolor of the oxochromium(VI)-amine complexes can account well for thechange of color from cream white to orange(red)-brown of the MOR-1-HAsorbent observed experimentally.

The regeneration of the MOR-1-HA columns by treating them withconcentrated HCl solutions (1.2-4 M) can be easily explained by theacidic hydrolysis of the (Ar—NH₂)CrO₃ complex [(Ar—NH₂)CrO₃]+H₂O→Ar—NH₃⁺+HOCrO₃ ⁻ with concomitant dimerization of HOCrO₃ to Cr₂O₇ ²⁻ (FIG.10). The exothermicity of the hydrolysis is predicted to be 30.5kcal/mol at the wB97XD/Def2-TZVPPD level. The estimated binding energyof the aniline ligand with the CrO₃ moiety was 34.4 kcal/mol, while thenegative natural atomic charge on the coordinated N donor atom rendersthe N atom susceptible to electrophilic attack by the H⁺ ions, which istransformed to ammonium NH₃ ⁻ salt, thus regenerating the MOR-1-HAcolumn.

EXPERIMENTAL SECTION

Synthesis of MOR-1

ZrCl₄ (0.625 gr, 2.7 mmol) and NH₂—H₂BDC (0.679 gr, 3.75 mmol) weredissolved in 75 mL DMF and 5 mL HCl in ajar. The jar was sealed andplaced in an oven operated at 120° C., remained undisturbed at thistemperature for 20 h, and then was allowed to cool at room temperature.White powder of MOR-1 was isolated by filtration and dried in the air.Yield: 1 g.

Synthesis of MOR-1-HA Composite

Method A.

0.1 g of sodium alginate was dissolved in 200 mL of warm water, and thesolution was allowed to cool. To the alginate solution 0.1 g of MOR-1was added. 0.1 g of CaCl₂ was then added into the alginate-MOR-1suspension with continuous stirring. The composite MOR-1-CA immediatelyprecipitated and was then isolated by filtration, washed with water andacetone and vacuum dried. To isolate the MOR-1-HA material, MOR-1-CA(0.2 g) was treated with 4 M HCl (50 mL) for ˜1 h. Yield: 0.85 g.

Method B.

This method is similar to Method A, with the difference that HClsolution (final concentration ˜4 M) was added to the alginate-MOR-1suspension instead of CaCl₂. Prior to the batch and column sorptionstudies, the MOR-1-HA was further treated with 4 M HCl to ensure fullprotonation of the MOR-1.

Method C.

ZrCl₄ (0.625 gr, 2.7 mmol) and NH₂—H₂BDC (0.679 gr, 3.75 mmol) weredissolved in 40 mL H₂O and 10 mL CH₃COOH in a round-bottom flask. Thesolution was heated under reflux conditions for ˜2 h. A fine suspensionof the MOR-1 formed and was allowed to cool. Then, SA solution (80 mL of0.05% SA water solution) was added to the suspension of MOR-1.Precipitation of the MOR-1-HA was immediately observed. To complete theprecipitation, HCl was added (final concentration ˜4 M). MOR-1-HA wasisolated by filtration, washed with water and acetone and vacuum dried.Yield ˜1 g. Thermal analysis data (in combination with EDS) indicatedthe formula [Zr₆(OH)₄O₄(NH₃C₈O₄H₃)₆]Cl₆.21H₂O-HA.

Preparation of the Column

50 mg of MOR-1-HA composite and 5 g of sand (50-70 mesh) was mixed in amortar and pestle and filled in a glass column.

Batch Ion-Exchange Studies

A typical ion-exchange experiment of MOR-1-HA with Cr₂O₇ ²⁻ was thefollowing: In a solution of K₂Cr₂O₇ (0.4 mmol, 117 mg) in water (10 mL,pH˜3), compound MOR-1-HA (0.04 mmol, 100 mg) was added as a solid. Themixture was kept under magnetic stirring for ≈1 h. Then, thepolycrystalline material, which had orange(red)-brown color, wasisolated by filtration, washed several times with water and acetone anddried in air.

The Cr(VI) uptake from solutions of various concentrations was studiedby the batch method at V:m ˜1000 mL/g, room temperature and 1 h contact.These data were used to determine Cr(VI) sorption isotherms. UV-Vis wasused for analysis of dichromate solutions with concentration ≥1 ppm. Thesolutions with Cr(VI) content less than 1 ppm were analyzed with ICP-MS.

The competitive and variable pH ion exchange experiments were alsocarried out with the batch method at V:m ratio (1000) mL/g, roomtemperature and 1 h contact.

To determine the sorption kinetics, Cr(VI) ion-exchange experiments ofvarious reaction times (1-60 min) have been performed. For eachexperiment, a 10 mL sample of Cr₂O₇ ²⁻ solution (initial dichromateconcentration=21.6 ppm, pH˜3) was added to each vial and the mixtureswere kept under magnetic stirring for the designated reaction times. Thesuspensions from the various reactions were filtrated and the resultingsolutions were analyzed for their chromium content with ICP-MS. Batchsorption studies for Se-containing solutions were performed similarly asfor Cr(VI).

Column Ion-Exchange Studies

Several bed volumes of the solution were passed through the column andcollected at the bottom in glass vials. The solutions with Cr₂O₇ ²⁻concentration ≥1 ppm were analyzed with UV-Vis, whereas the Cr contentof those with smaller concentration was determined with ICP-MS. Columnsorption studies for Se-containing solution were performed similarly asfor Cr(VI).

Physical Measurements

PXRD diffraction patterns were recorded on a Bruker D8 Advance X-raydiffractometer (CuKa radiation, λ=1.5418 Å). Energy DispersiveSpectroscopy (EDS) were performed using a JEOL JSM-6400V scanningelectron microscope (SEM) equipped with a Tracor Northern EDS detector.Data acquisition was performed with an accelerating voltage of 25 kV and40 s accumulation time. Thermogravimetric analysis (TGA) was carried outwith a Shimatzu TGA 50. Samples (10±0.5 mg) were placed in a quartzcrucible. X-ray photoelectron spectroscopy was performed on a PerkinElmer Phi 5400 ESCA system equipped with a Mg Kα x-ray source. Sampleswere analyzed at pressures between 10⁻⁹ and 10⁻⁸ Torr with a pass energyof 29.35 eV and a take-off angle of 45°. All peaks were referred to thesignature C_(1s) peak for adventitious carbon at 284.6 eV. The peakswere fitted by using the software XPSPEAK41. UV/vis Cr(VI) solutionspectra were obtained on a Shimadzu 1200 PC in the wavelength range of200-800 nm. IR spectra were recorded on KBr pellets in the 4000-400 cm⁻¹range using a Perkin-Elmer Spectrum GX spectrometer. Gas sorptionisotherms were measured on a Quantachrome NOVA 3200e volumetricanalyzer. The solutions with Cr(VI) content less than 1 ppm wereanalyzed with Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS)using a computer-controlled Thermo Fisher X Series II InductivelyCoupled Plasma Mass Spectrometer with a quadruple set-up equipped withCollision Cell Technology.

Example 2

This example reports a new synthetic method for the isolation of purelymicroporous and highly crystalline MOR-1 and its composite formMOR-1-HA, which involves acidified water as a solvent and is completedwithin an hour. (The structure of (protonated) MOR-1 is shown in FIG.11.) The obtained materials show exceptional capability to absorbhexavalent chromium under a diverse range of experimental conditions,including pH of the solution and the presence of competitive ions.Importantly, the composite MOR-1-HA is particularly suitable to be usedin an ion-exchange column, showing excellent Cr(VI) absorptionproperties. In addition, for the first time it is shown that theMOR-1-HA column is efficient for the decontamination of industrial(chrome plating) Cr(VI) wastewater samples. Considering the relativelylow cost, and the fast and environmentally friendly synthesis method ofthe MOR-1-HA reported here, the MOR-1-HA column seems promising forreal-world applications in the field of environmental remediation.

Results and Discussion

Synthesis of MOR-1 and MOR-1-HA Composite

In Example 1, MOR-1-HA composite was prepared via a three-step procedureinvolving: a) synthesis of MOR-1 via a solvothermal reaction in DMF/HClsolution; b) encapsulation of MOR-1 by calcium alginate (CA) resultingin a MOR-1-CA composite; and c) formation of MOR-1-HA material bytreatment of MOR-1-CA with HCl acid. Alternatively, the MOR-1-HA couldbe prepared by addition of HCl acid in a water suspension of MOR-1 andsodium alginate (SA). As mentioned above, the material would be moreattractive for applications if it could be synthesized with a fast andinexpensive synthesis method involving minimal quantities of organicsolvents. Recently, it was shown that a UiO-66-amino functionalized typemetal organic framework could be isolated with a reflux reaction ofalmost equimolar Zr(NO₃)₄ and NH₂.H₂BDC (2-amino-terephthalic acid) inwater-acetic acid solutions. (Z. Hu, Y. Peng, Z. Kang, Y. Qian, and D.Zhao, Inorg. Chem., 2015, 54, 4862.) However, the reported materialshowed structural characteristics that differ from those of the compoundisolated from the reaction with DMF (J. H. Cavka, S. Jakobsen, U.Olsbye, N. Guillou, C. Lamberti, S. Bordiga and K. P. Lillerud, J. Am.Chem. Soc., 2008, 130, 13850; M. J. Katz, Z. J. Brown, Y. J. Colon, P.W. Siu, K. A. Scheidt, R. Q. Snurr, J. T. Hupp and O. K. Farha, Chem.Commun., 2013, 49, 9449; M. Kandiah, M. H. Nilsen, S. Usseglio, S.Jakobsen, U. Olsbye, M. Tilset, C. Larabi, E. A. Quadrelli, F. Boninoand K. P. Lillerud, Chem. Mater., 2010, 22, 6632). It was thuschallenging to isolate hydrothermally a UiO-66-amino functionalizedmaterial (i.e. MOR-1) with the same features as those of the well-knownmaterial prepared with solvothermal reaction. This synthesis wasattempted by modifying the reported reflux synthesis method (FIG. 12).Thus, the same Zr⁴⁺ source (i.e. ZrCl₄) and a ligand to metal salt molarratio (˜1.4) equal to those used in the solvothermal synthesis ofUiO-66-NH₂BDC were employed, with the difference that in our synthesisthe solvent was a mixture of water and acetic acid (25 v/v % aceticacid). This reaction resulted in a fine suspension of the MOR which wasformed in less than 1 h. The product of this reaction, isolated viacentrifugation, contained impurities, probably some amount of unreactedorganic ligand. Indeed, treating the product with HCl acid solution,which can dissolve the NH₂—H₂BDC ligand, resulted in the isolation ofpure MOR-1 in ˜70% yield. After the method for the isolation of the highquality UiO-66 amino-functionalized material was established, the nextstep was the preparation of the MOR-1-HA composite. Fortunately, theisolation of the composite did not involve the separation process viacentrifugation followed for MOR-1, which is time-consuming and notattractive for large scale synthesis of materials. The isolation ofmaterials forming colloidal solutions requires the use of flocculationagent that causes agglomeration of the particles thus simplifying theseparation process. Such a flocculation agent could be sodium alginate,which in an acidic environment is transformed to alginic acid. Thelatter forms an insoluble polymer shell around MOR particulates,resulting in the precipitation-easy separation of the solid from thesolution (FIG. 12). Indeed, by adding sodium alginate to the MOR-1water-acetic acid suspension, the MOR-1-HA was readily precipitated andcould be isolated via simple filtration. Only a small amount of sodiumalginate was required for the isolation of the composite material, andthus the composite showed almost identical properties to those of thepristine MOR material. Specifically, the composite isolated containedalginic acid in an amount up to 2.1% wt. (see experimental section,supporting information). The obtained product was further treated withHCl acid to dissolve the unreacted NH₂—H₂BDC ligand and complete theprotonation of the amino-functional groups of the material. Studies forthe formation of the material vs. the reaction time were also performed.The results indicated that a) a significant amount of MOR-1-HA wasformed within only 5 min; and b) an hour of reflux reaction was enoughto achieve the maximum possible yield for the isolation of the MOR-1-HAcomposite material.

Characterization of MOR-1 and MOR-HA Materials

Field Emission-Scanning electronic microscopy (FE-EM) images showed thatboth the MOR-1 and MOR-1-HA materials were composed of aggregatedpolyhedral-shape nanoparticles with size ˜150-300 nm. (FIG. 13).High-magnification images revealed that the nanoparticles of MOR-1 werespongy with relatively large voids, whereas those of MOR-1-HA containedsignificantly smaller pores in their surface. Presumably, this was dueto the fact that a thin layer of alginic acid covered the large pores inthe surface of MOR-1 nanoparticles, thus creating the densernanoparticles of the composite. Therefore, MOR-1-HA isolated in arelatively compact form was much less dispersed in water, and thus couldbe successfully utilized in columns (see below). Note that no clearshape and size of nanoparticles could be observed by SEM studies ofMOR-1 isolated from the solvothermal reaction in DMF and thecorresponding MOR-1-HA material. Thus, in this case, differences inmorphology between MOR-1 and MOR-1-HA particles could not be observed.Here, for the first time, the clear differences between MOR-1 andMOR-1-HA materials could be visualized, which may explain theirdifferent capability to form or not fine suspension in water.

Powder X-ray diffraction (PXRD) studies indicated that MOR-1 isolatedwith the 1 h reflux reaction after its purification with HCl acid showsthe typical structure of UiO-66 type materials (FIG. 14A. Elemental(C,H,N), energy-dispersive spectroscopy (EDS) (indicating Zr:Cl molarratio ˜1) and thermal analysis (TGA) data indicated the formula[Zr₆O₄(OH)₄(NH₃ ⁺-BDC)₆]Cl₆.35H₂O for MOR-1. PXRD (FIG. 14A) and EDS(revealing Zr:Cl molar ratio ˜1) data for the composite sample confirmedits close similarity to the pristine MOR-1 solid. TGA data were alsoused for the determination of the water content of composite material(˜19 water molecules).

Nitrogen physisorption measurements recorded at 77 K for the activatedMOR-1 and MOR-1-HA revealed type-I adsorption isotherms, characteristicof microporous solids (FIG. 14B). The Brunauer-Emmett-Teller (BET)surface areas of the MOR-1 and MOR-1-HA were determined to be 1097(Langmuir 1638 m²/g) and 1182 (Langmuir 1670 m²/g) m²/g respectively.These values fall within the range of surface areas found foramino-functionalized UiO-66 type materials prepared with solvothermalreactions (J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti,S. Bordiga and K. P. Lillerud, J. Am. Chem. Soc., 2008, 130, 13850; M.J. Katz, Z. J. Brown, Y. J. Colon, P. W. Siu, K. A. Scheidt, R. Q.Snurr, J. T. Hupp and O. K. Farha, Chem. Commun., 2013, 49, 9449; M.Kandiah, M. H. Nilsen, S. Usseglio, S. Jakobsen, U. Olsbye, M. Tilset,C. Larabi, E. A. Quadrelli, F. Bonino and K. P. Lillerud, Chem. Mater.,2010, 22, 6632.) CO₂ adsorption isotherms at 1 bar and 273 K indicated asorption capacity of ˜4.4 mmol/g for both samples. Analysis of CO₂adsorption data with the density function theory (DFT) suggested thatboth MOR-1 and MOR-1-HA had a microporous network with pore size in therange of 8-9 Å. Interestingly, the MOR-1 and MOR-1-HA polymers presentedhere showed significantly higher surface area and CO₂ sorption capacitycompared to those (BET=833 m²/g, Langmuir=1073 m²/g; CO₂ sorptioncapacity=2.8 mmol/g at 273 K) for the reported UiO-66amino-functionalized compound prepared by a hydrothermal reaction. (Z.Hu, Y. Peng, Z. Kang, Y. Qian, and D. Zhao, Inorg. Chem., 2015, 54,4862.) Furthermore, the type-I shape for the isotherms of MOR-1 andMOR-1-HA indicated a predominantly microporous structure, whereas thereported UiO-66-NH₂BDC solid isolated hydrothermally showed acombination of type-I and type-IV isotherms revealing the existence ofboth micro- and mesoporosity. Thus, here is proposed for the first timea hydrothermal synthesis approach that yields amino-functionalizedUiO-66-type materials with same characteristics as those found in theUiO-66-NH₂BDC compound prepared with typical solvothermal reaction.

Isolation and Characterization of Cr(VI)-Containing Material

The isolation of the Cr(VI)-loaded composite material [MOR-1-HA@Cr(VI)]was achieved by treating it with a Cr₂O₇ ²⁻ water solution for ˜1 h. Ananion-exchange reaction is taking place represented by the followingequation:[Zr₆O₄(OH)₄(NH₃ ⁺-BDC)₆]Cl₆-HA+3Cr₂O₇ ²⁻→[Zr₆O₄(OH)₄(NH₃⁺-BDC)₆](Cr₂O₇)₃—HA+6Cl⁻  (1)

EDS analysis showed no Cl in the Cr(VI)-exchanged material. Furthermore,analytical data from ICP-MS, EDS and UV-Vis spectroscopy indicated aZr:Cr molar ratio of ˜1, consistent with the replacement of 6 Cl⁻ by 3Cr₂O₇ ²⁻ anions. PXRD data of MOR-1-HA@Cr(VI) indicated that thestructure of the UiO-66 type framework was preserved after theion-exchange process (FIG. 14A). The presence of Cr(VI) ions was alsoevidenced by IR, which showed a characteristic peak at 924 cm⁻¹attributed to the anti-symmetric Cr^(VI)O₃— stretch. Furthermore, XPSdata showed the presence of Cr2p_(1/2) and Cr2p_(3/2) core-levelsignals. The main components of these peaks corresponded to bindingenergies (588.1 and 579.2 eV), which are characteristic of Cr in the(VI) valence state. The insertion of Cr(VI) species into the pores wasalso demonstrated by the substantially smaller BET surface area ofCr(VI) exchanged samples compared to that of pristine compositematerial. Thus, after the Cr(VI) exchange process, the surface areadropped from 1182 m²/g for MOR-1-HA to 298 m²/g for MOR-1-HA@Cr(VI)(FIG. 14B).

Finally, FE-SEM studies indicated that the nanoparticles ofMOR-1-HA@Cr(VI) retained the polyhedral shape of the MOR-1-HA particles(FIG. 15). However, the MOR-1-HA@Cr(VI) nanoparticles contained somedefects and relatively large pores in their surface, which presumablyresulted from the ion-exchange process.

Batch Ion Exchange Studies

Ion-Exchange Isotherm Data

Cr(VI) equilibrium ion-exchange studies for MOR-1-HA were performed atpH˜3, in order to reproduce the usual acidic conditions ofCr(VI)-bearing industrial wastewater. Under such conditions, the mainform of Cr(VI) was Cr₂O₇ ²⁻, with some contribution from HCrO₄ at diluteCr(VI) acidic solutions.

Fitting of the isotherm data with the Langmuir equation (FIG. 16)revealed a maximum sorption capacity of 280±19 mg Cr₂O₇ ²/g of MOR-1-HA,which corresponded to a capacity of 286.0±19.4 Cr₂O₇ ²⁻ mg/g of MOR-1,considering that the composite contains ˜97.9% MOR-1. This sorptioncapacity was consistent with the absorption of 3.1±0.2 moles of Cr₂O₇ ²⁻per formula unit of the MOR ([Zr₆O₄(OH)₄(NH₃ ⁺-BDC)₆]Cl₆.xH₂O, x˜19 forthe MOR component of the composite), which was close to its maximumsorption capacity (3 moles per formula unit). Fitting of the isothermdata could also be done using the Freundlich model.

The high efficiency of the composite for dichromate sorption was alsorevealed by the values of the distribution coefficient K_(d) calculatedby the following equation:

$\begin{matrix}{K_{d} = \frac{V\left\lbrack {\left( {C_{0} - C_{f}} \right)/C_{f}} \right\rbrack}{m}} & (2)\end{matrix}$

where C₀ and C_(f) are the initial and equilibrium concentrations ofCr₂O₇ ²⁻ (ppm) respectively, V is the volume (ml) of the testingsolution and m is the amount of the ion exchanger (g) used in theexperiment. The maximum K_(d) ^(Cr) ₂ ^(O) ₇ values obtained from thebatch equilibrium studies are in the range 4.5×10⁴-1.2×10⁵ mL/g. Suchvalues are considered excellent and indicate the exceptional affinity ofthe material for Cr(VI). The material could be also regenerated andreused for Cr(VI) sorption, as indicated by the column sorption studies(see below).

The ion-exchange studies focused on the composite and not on thepristine MOR, since only the composite form is suitable for columnion-exchange (see below). For comparison, however, the isotherm Cr₂O₇ ²⁻sorption data for the MOR-1 material has also been determined). Theresults revealed a maximum sorption capacity of 321±16 Cr₂O₇ ²⁻ mg/g ofMOR-1, slightly higher than that found for the composite. Thus, thepresence of alginic acid in such a small quantity (˜2% wt.) in thecomposite resulted in a minor differentiation of the ion-exchangeproperties of the metal organic material.

Kinetic studies. The kinetics of the Cr₂O₇ ²⁻ absorption by MOR-1-HAwere also investigated. FIG. 17A shows UV-Vis data from the kineticexperiments (initial dichromate concentration=21.2 ppm, pH˜3). FIG. 17Bshows % Total Cr removal by MOR-1-HA vs. time (min). The resultsindicated that this sorption process was quite fast, with ˜85.5% of theinitial Cr₂O₇ ²⁻ content (C₀=21.2 ppm, pH˜3) removed within only 1 minMOR-1-HA/solution contact. After 3 min, the Cr₂O₇ ²⁻ removal percentageincreased to 97.2%, whereas the ion exchange reached its equilibriumwithin 9 min, and more than 99% Cr₂O₇ ²⁻ sorption was observed. Thekinetic data could be roughly fitted with the Lagergren's first orderequation:q _(t) =q _(e)[1−exp(−K _(L) t)]  (3)

where q_(e)=the amount (mg/g) of metal ion absorbed in equilibrium andK_(L)=the Lagergren or first-order rate constant (Fitting parameters:q_(e)=20.6±0.4 mg/g, K_(L)=1.9±0.3 min⁻¹).

The rapid Cr(VI) sorption kinetics observed for MOR-1-HA resulted fromits highly porous structure, allowing fast diffusion of ions within thepores and the strong Cr(VI)-amine groups interactions.

Variable pH studies. Although more focus was placed on the Cr(VI)sorption under acidic conditions that are usually observed in industrialwaste, also studied was the Cr(VI) ion exchange by MOR-1-HA in arelatively wide pH range (1-8). The results indicated that the materialwas capable of absorbing Cr(VI) from highly acidic to alkaline solutions(FIG. 18). In particular, it showed 97-99.6% Cr(VI) removal capacitiesin the pH range 2-8 (initial total chromium concentration=10.2 ppm),whereas even at pH˜1, MOR-1-HA displays a high Cr(VI) removal capacity(˜82%).

Selectivity Studies

Competitive Cr₂O₇ ²⁻/Cl⁻, Cr₂O₇ ²⁻/Br⁻, Cr₂O₇ ²⁻/NO₃ ⁻ and Cr₂O₇ ²⁻/SO₄²⁻ sorption experiments were also performed for MOR-1-HA. Cl⁻, Br⁻ andNO₃ ⁻ ions had no effect on the dichromate (initial concentration 0.25mM, pH˜3) sorption by MOR-1-HA. Thus, high dichromate removal capacities˜98-99% and excellent K_(d) ^(Cr) ₂ ^(O) ₇˜1.0-2.2×10⁵ mL/g wereobserved even in the presence of a large excess of Cl⁻, Br⁻ and NO₃ ⁻ions (Cl⁻, Br⁻, NO₃ ⁻ concentration=2.5 mM). SO₄ ²⁻, as a divalentcation, was expected to be a stronger competitor for dichromate ionexchange. Still, even in the presence of ten-fold excess of SO₄ ²⁻ ions(concentration=2.5 mM), a significant dichromate removal capacity (˜55%)was obtained. The selectivity of MOR-1-HA for dichromate anions wasexplained on the basis of strong O₃Cr^(VI) . . . NH₂ interactions.

Column Ion Exchange Studies

Initial Check of the Sorbents.

The next step was the study of the column ion-exchange properties of thematerials. The stationary phase in the columns prepared was a mixture ofthe sorbent and sand, an inexpensive and inert material. The use of sucha mixture instead of the pure sorbent material ensured a continuous flowof the water through the column, since a) the sorbent particles wereimmobilized and separated by sand pieces; and b) the pressure exerted bywater was absorbed mainly by the sand, thus keeping the sorbentparticles intact. Columns containing a sorbent to sand mass ratio of1:100 proved to be highly effective for Cr(VI) sorption and also showeda stable and relatively fast water flow (see below and reference 3).That columns with high efficiency for Cr(VI) removal contained only asmall quantity of the sorbent (˜1% by weight), and the main component(99% by weight) of the stationary phase is sand, an abundant and verylow cost material, are both economically attractive features.

Prior to the breakthrough sorption experiments, MOR-1 and MOR-1-HA/sandcolumns (with a sorbent to sand mass ratio of 1:100) were tested todetermine the capability of the sorbent to remain fixed in thestationary phase, as well as the flow rate for the column. SEM studies,presented above, indicated that the MOR-1 particles were porous andspongy, and thus it was expected to be easily dispersed in water.Indeed, MOR-1 (even mixed with an inert material as sand) was graduallyremoved from the column, since it formed a fine water suspension. Thus,clearly MOR-1 was not suitable to be used for column sorptionapplications. The composite material, however, was composed ofrelatively compact MOR particles partially coated by the insolublealginic acid shell (see SEM images above), and thus it had limitedcapability to form suspensions in water. As a result, the effluentsflowing out of the composite/sand columns were clear solutions.

Flow rate for the columns should be also taken into account. Sorbentsthat result in column clogging are not desirable for applications. Thus,the flow rate of a MOR-1-HA/sand column was investigated. The water flowthrough the MOR-1-HA/sand column was observed to be stable over severalruns and relatively fast (1.2-1.4 mL/min). MOR-1-HA particles were ofuniform (polyhedral) shape (FIG. 14), and thus they could be distributedevenly in the column allowing a continuous and stable water flow. Thus,the MOR-1-HA/sand column seems promising in terms of immobilization ofthe sorbent in the stationary phase and flow rate.

Determination of Breakthrough Curves and Sorption Capacity.

Sorption experiments with the MOR-1-HA/sand(mass ratio 1:100) column andinitial Cr₂O₇ ²⁻ concentration of 6.4 ppm (pH˜3) revealed that 97 bedvolumes (Bed volume=bed height (cm)×cross sectional area (cm²)=3.5 mL)of the effluent samples (bed volumes) showed a total Cr content ≤30 ppb,significantly below the acceptable safety limits defined by the US EPA(100 ppb) and EU (50 ppb) (FIGS. 19A and 19B). The column could beregenerated by washing it with HCl acid solution (4M). The regenerationprocess could be visually observed by the decolorization of the(yellow-colored) Cr(VI)-loaded column. After regeneration, the columncould be reused, showing only a small decrease (˜4) of bed volumes, withCr content below the Cr safety limits. Even after a fifth run the columndisplayed a high number of bed volumes (84) with total Cr concentrations30 ppb. The breakthrough capacity Q_(b) (mg) of this ion-exchange columncould be determined by the equation:Q _(b) =C ₀ ·V _(b)  (4)where C₀ is the initial concentration of Cr₂O₇ ²⁻ (mg/L), and V_(b) isthe volume (L) passed until the breakpoint concentration (usuallydefined as the maximum acceptable concentration of the contaminant).

The numbers of bed volumes passed through the column until thebreak-point concentration (i.e., total Cr concentration ≤50 ppb) were97, 93, 87, 92 and 84 for the 1^(st)-5^(th) column runs, respectively.Thus, the Q_(b) values for the different runs of the specificMOR-1-HA/sand column and Cr₂O₇ ²⁻ initial concentration of 6.4 ppm werecalculated 2.17 (1^(st) run), 2.08 (2^(nd) run), 1.95 (3^(rd) run), 2.06(4^(th) run) and 1.88 (5^(th) run) mg Cr₂O₇ ²⁻ (FIG. 19).

The effect of the initial Cr(VI) concentration on the breakthroughsorption capacity of this MOR-1-HA/sand column was also examined. It wasobserved that breakthrough capacities of 2.06-2.25 and 2.25-2.34 mgCr₂O₇ ²⁻ were obtained for initial dichromate concentrations of 53.5 and25.7 ppm, respectively. Thus, similar breakthrough capacity was observedindependently of the initial Cr(VI) concentration, thus emphasizing thereproducible sorption results obtained with the MOR-1-HA/sand column.

In addition, the performance of the MOR-1-HA/sand column was tested forthe decontamination of solutions containing low Cr levels. However,these were above the safety limits. Thus, 1.1 L (˜315 bed volumes) of asolution with a total Cr concentration of 394 ppb (pH˜3) was passedthrough the MOR-1-HA/sand column. ICP-MS analysis for the Cr content ofthe effluents collected indicated that the Cr concentrations were 7-27ppb (FIG. 20), which was well below the EU and US EPA acceptable limits.These results indicated the exceptional capability of the MOR-1-HA/sandcolumn to remediate water contaminated with extremely low Cr levels.Note that it is not easy to treat wastewater with quite low Crconcentrations (<1000 ppb) with common methods such as precipitation.Thus, the development of new technologies that are effective for suchlow Cr levels is particularly desirable.

The performance of MOR-1-HA/sand column (MOR-1-HA to sand massratio=1/100), containing MOR-1-HA prepared with the reflux synthesis/SAaddition, may be compared to that of the corresponding column withcomposite isolated from solvothermally prepared MOR-1. The latterdisplayed breakthrough capacity of 1.55-1.68 mg Cr₂O₇ ²⁻ (number of bedvolume till the breakpoint concentration=74-80, 1 bed volume=3.5 mL,initial Cr₂O₇ ²⁻ concentration=6 ppm), lower than that (1.88-2.17 mgCr₂O₇ ²⁻ for initial dichromate concentration of 6.4 ppm) of the columnwith MOR-1-HA isolated from reflux synthesis-SA addition. Thus, highquality MOR-1-HA could be prepared with a fast, low-cost andenvironmentally-friendly synthesis that, at the same time, showedimproved column Cr(VI) sorption properties.

Column Sorption of Chrome Plating Wastewater

A common type of Cr(VI)-containing industrial waste is chrome platingwastewater. Such Cr(VI)-bearing water is generated by rinsing the platedparts upon their removal from the plating bath. It should be noted thatthe Cr(VI) concentration in metal plating effluents may vary from veryhigh to moderate or low levels depending on the amount of water used inthe rinsing step of the metal plating process. Thus, Cr(VI)concentrations greater than 1000 and lower than 10 ppm have beenreported for metal plating waste. Encouraged by the above excellentcolumn sorption results, it was decided to test the performance of thecolumn for the removal of Cr(VI) from chrome plating wastewater. A metalplating company (located in Northern Greece) provided us with twodifferent types of hexavalent chromium waste: Sample (A) containeddichromate ions in very high concentrations (4855 ppm, pH˜1.6); andsample (B) was a neutral pH solution with lower Cr(VI) content (analysisof the Cr(VI) concentration of this solution was done after adjustingits pH to ˜3, see below).

Sample A was too concentrated to be treated with our laboratory-scaleion exchange columns. Thus, Cr(VI)-contaminated wastewater was preparedby diluting the original sample A to ˜54 ppm of Cr₂O₇ ²⁻ (pH˜3.5 afterthe dilution). The column was very efficient in decontaminating thiswastewater, something that could be seen even with the naked eye. Thebreakthrough curves (FIG. 21A) obtained from five column runs (withregeneration of the column after each run) indicated 13-14 bed volumeswith total Cr concentration <50 ppb (EU defined acceptable Cr limit),and a breakthrough capacity of 2.44-2.63 mg Cr₂O₇ ²⁻ (FIG. 21B). Thesebreakthrough capacity values were similar to those obtained for theexperiments with the laboratory prepared dichromate solutions.

Also studied were the column ion-exchange properties with sample Bsupplied by the metal plating company. Prior the sorption experiments,the pH of sample B was adjusted to ˜3 in order to enable the UV-Visanalysis of Cr(VI) as Cr₂O₇ ²⁻ anions (at neutral pH there isequilibrium between chromate and dichromate anions) and allow acomparison with the results for the synthetic dichromate solutions. TheUV-Vis data for sample B revealed a concentration of Cr₂O₇ ²⁻ of 108ppm. The decontamination of the wastewater sample after its treatmentwith the MOR-1-HA/sand column was apparent even with the naked eye. Fiveruns of the column with sample B (FIG. 22A) revealed breakthroughcapacities of 2.268-2.646 mg, relatively close to those observed for theexperiments with sample A and the laboratory-prepared solutions. Inthese column ion-exchange experiments, due to the relatively highinitial Cr(VI) concentration of sample B, the sorbent rapidly reachedcomplete saturation with Cr(VI).

The MOR-1-HA/sand column performance for the decontamination of sample Bcould be very well modelled by the Thomas equation:

$\begin{matrix}{\frac{C}{C_{0}} = \frac{1}{1 + {\exp\left( {\frac{k_{Th}}{Q}\left( {{q_{\max}m} - {C_{0}V_{eff}}} \right)} \right)}}} & (5)\end{matrix}$where C, C₀ are the concentrations (mg/L) of the ion in the effluent andits initial concentration (mg/L), respectively; k_(Th) (L mg⁻¹ min⁻¹) isthe Thomas model or sorption rate constant; q_(max)(mg/g) is thepredicted maximum sorption capacity; m(mg) is the mass of the sorbent;and Q (mL min⁻¹) is the volumetric flow and V_(eff) is the effluentvolume (mL). (V. J. Inglezakis and S. G Poulopoulos, Adsorption, IonExchange and Catalysis. Design of Operations and EnvironmentalApplications, Elsevier, 2006.) The fitting of the data with the Thomasequation (FIG. 22B) revealed maximum column sorption capacities of 61-63mg Cr₂O₇ ²⁻/g of the sorbent. These predicted column sorption capacitieswere close to those experimentally observed (67-70 mg/g). The latterwere calculated from the difference between the Cr₂O₇ ²⁻ content of theinitial and effluent solutions. The excellent description of the columnion exchange generated using the Thomas model indicated that theexternal (fluid-film) and intra-particle mass transfer resistance hadnegligible effect on the column ion-exchange of the MOR-1-HA sorbent.

Finally, another important feature of the breakthrough curve was thedegree of column utilization defined as the ratio of breakthrough tototal column sorption capacity. For practical applications, it isdesirable to achieve a degree of column utilization as close as possibleto unity. For the column ion exchange experiments with wastewater sampleB, the degree of column utilization (%) lay in the range of 78-89%, thusrevealing the highly efficient performance of the MOR-1-HA sand column.

Conclusions

In conclusion, a new rapid, green and low-cost synthetic method forMOR-1 and composite MOR-1-HA materials was developed, which involved 1 hreflux reaction in water-acetic solvent (yielding MOR-1) and addition ofsodium alginate (SA) to the resulted suspension of MOR-1 under refluxconditions (yielding MOR-1-HA). The hydrothermal synthesis approachused, for the first time, afforded UiO-66 type amino-functionalizedmaterials which were purely microporous as the well-known UiO-66-NH₂BDCcompound prepared with the solvothermal method. It is important thathigh quality UiO-66 amino-functionalized compounds could be preparedvery fast via an inexpensive reflux synthesis in acidified water, sincesuch materials are of great interest not only for their Cr(VI) sorptioncapacity but also for their gas sorption and photocatalytic properties,post-synthetic chemistry etc. Furthermore, the presented syntheticmethod revealed the usefulness of sodium alginate (SA), being a) theprecursor for the formation of the alginic acid component of thecomposite; and b) a flocculation agent for the easy separation of MOR-1from its fine suspension in water. Thus, SA could be useful for thelarge scale synthesis and facile isolation of a number of metal organicmaterials forming colloidal water solutions, such as various UiO-66,UiO-67 analogues, MOFs of the MIL-family, etc.

Detailed batch Cr(VI) sorption studies for the MOR-1-HA compositeisolated with the new method revealed its exceptional capability toabsorb Cr(VI) under various conditions. This sorbent was particularlyable to be used in ion-exchange columns. It comprised of relativelycompact and polyhedral shape nanoparticles that could be uniformlydistributed in the column allowing a stable flow rate. In addition, thissorbent, due to the coating of MOR-1 particles by alginic acid, was noteasily dispersed in water (in contrast to the pristine MOR-1 material),and thus could be immobilized in the stationary phase of the column.Thus, an ion-exchange column containing MOR-1-HA showed relatively highand reproducible Cr(VI) sorption capacities, as well as excellentregeneration capability and reusability. Compared to columns withcomposites isolated from solvothermally prepared MOR-1, the column withMOR-1-HA synthesized with the new method exhibited improved performance.Importantly, this column was highly efficient for the removal of Cr(VI)not only from laboratory prepared solutions, but also from industrialwastewater samples. Overall, the results indicated that MOR-1-HAion-exchange column could be inexpensive, considering the relatively lowcost of the new synthetic method for MOR-1-HA, and also promising forthe remediation of real-world wastewater. The next step of this researchcould be thus the development of large scale MOR-1-HA columns and theirapplication in wastewater treatment plants.

Example 3

This example reports a new member of the UiO-66 series, namely the MOF[Zr₆O₄(OH)₈(H₂O)₄(H₂PATP)₄]Cl₈.12H₂O (MOR-2)(H₂PATP=2-((pyridin-1-ium-2-ylmethyl)ammonio)terephthalate). MOR-2showed exceptionally high dichromate sorption capacity (˜402 mg/g) andremarkably rapid sorption kinetics (equilibrium is reached within 1min). MOR-2 also exhibited excellent chromate sorption capacity (˜264mg/g) and rapid uptake of this Cr(VI) species. MOR-2 was capable ofeliminating Cr(VI) from a variety of solutions including industrial anddrinking water samples. MOR-2-alginic acid (MOR-2-HA) composite is alsodescribed herein. This material was successfully employed inion-exchange columns that showed very efficient performance for thesorption of both high and trace amounts of Cr(VI) from simulated andindustrial waste samples, with noticeable recyclability of the column.Furthermore, the ability of MOR-2 toward selective and real-timeluminescence sensing of ppb levels of Cr(VI) in real-world water sampleswas demonstrated.

Results and Discussion

Synthesis of MOR-2 and MOR-2-HA

MOR-2 was synthesized via a solvothermal reaction of ZrCl₄ and the newligand H₂PATP (=2-((pyridin-2-ylmethyl)amino)terephthalic acid, FIG. 23)in DMF-HCl solution. To introduce positive charge on the framework ofMOR-2 and thus unlock its anion exchange properties, the material wastreated with relatively concentrated HCl solution (4 M) in order toprotonate its pyridine and amine moieties, affording pyridinium andammonium functional groups charge balanced by Cl⁻ anions. Thepyridinium-methyl-ammonium moieties were expected to be particularlycapable of strongly binding Cr(VI) via the formation of both covalentand hydrogen bonds (see below).

Pristine MOR-2 is not suitable for use as a stationary-phase inion-exchange columns since it is isolated as a very fine powder whichpasses through the frits into the eluding fractions (see below). Totackle this problem, the composite of MOR-2 was prepared with alginicacid, MOR-2-HA, by adding HCl into a suspension of MOR-2 in a sodiumalginate (SA) solution in water. The composite material could be easilyseparated from water because a thin HA shell covered the MOR particles,which were now far less prone to form fine suspensions in aqueoussolutions. In contrast, as-prepared MOR-2 forms very fine suspensionupon its contact with water. Note that the composite can be successfullyisolated (and used in ion-exchange columns) using only 1% wt. ofalginate. Thus, the particles of MOR-2 were not covered by a thick layerof alginic acid that could slow the sorption kinetics.

Characterization of MOR-2 and MOR-2-HA

Field-emission scanning electron microscopy (FE-SEM) studies revealedthat the MOR-2 was composed of sponge-like aggregates of particles. Incontrast, due to the alginic acid coating, the particles of MOR-2-HAwere more compact and, as a consequence, were less dispersed in water.Powder X-ray diffraction (PXRD) data indicated the structural similarityof MOR-2 (and MOR-2-HA) with the UiO-66 MOF.

Nevertheless, a series of analytical data (C,H,N, EDS, Zr analyses andTGA) were consistent with the formula H₁₆[Zr₆O₁₆(H₂PATP)₄]Cl₈.12H₂O forthe MOR-2 material. Nitrogen physisorption measurements carried out at77 K for the activated MOR-2 and MOR-2-HA samples showed typical type-Iadsorption isotherms, characteristic of microporous solids. TheBrunauer-Emmett-Teller (BET) surface areas of the MOR-2 and MOR-2-HAwere measured to be 354 and 442 m² g⁻¹ respectively. CO₂ adsorptionisotherms recorded at 1 bar and 273 K indicated a sorption capacity of˜0.94 and 1.66 mmol g⁻¹ for MOR-2 and MOR-2-HA respectively. Analysis ofCO₂ adsorption data with the density functional theory (DFT) suggeststhat both MOR-2 and MOR-2-HA have a microporous network with a pore sizeof about 5.5 Å.

Isolation and Characterization of Cr(VI)-Loaded Materials

The porous structure of MOR-2 in combination with the presence of labileCl⁻ anions and pyridinium-methyl-ammonium functional groups (which areexpected to show high affinity for Cr(VI)) motivated the study of theCr(VI) anion exchange properties of this material. The sorption ofdichromate and chromate species by MOR-2 can be visually observed by thechange of the color of the pristine compound (from light yellow toorange-brown and yellow-brown after the sorption of Cr₂O₇ ²⁻ and CrO₄²⁻, respectively). EDS analytical data revealed that no Cl⁻ existed inthe Cr(VI)-exchanged products. Furthermore, various analytical (EDS,ICP-MS and UV-Vis) data indicated the presence of 9 and 6 Cr(VI) performula unit of dichromate and chromate-loaded MOR-2, respectively. Themechanism of the anion exchange processes is discussed below.

PXRD data indicated that the crystal structure of MOR-2 was retainedafter the Cr(VI) exchange processes. However, both dichromate andchromate loaded materials showed a BET surface area of ˜23 m²/g,substantially lower than that of pristine MOR-2. This result confirmedthe incorporation of the Cr(VI) species into the pores of the material.The IR spectra of the Cr(VI)-containing compounds exhibited acharacteristic peak at ˜926 cm⁻¹ (not existing in the IR spectrum of thepristine material) attributed to the anti-symmetric Cr^(VI)O₃-stretch.X-ray photoelectron spectroscopy (XPS) data showed Cr2p_(1/2) andCr2p_(3/2) core-level signals, with their main components correspondingto binding energies of ˜589 and 580 eV, respectively. These values wereconsistent with hexavalent chromium species.

Batch Ion Exchange Studies

Batch ion exchange experiments were performed in order to gain furtherinsight into the Cr(VI) sorption properties of MOR-2. Both dichromateand chromate ion exchange properties of MOR-2 are presented.

Cr₂O²⁻ Exchange

Determination of Isotherm

Ion-exchange experiments using relatively low (˜20 ppm) to extremelyhigh (up to 3700 ppm) initial dichromate concentration were carried out.Note that such a wide range of Cr(VI) concentrations is commonlyobserved in industrial effluents (see below). The isotherm dichromateion exchange data, obtained at pH˜3, are shown in FIG. 24A.

These data can be described with the Langmuir model (eq. 1)

$\begin{matrix}{q = {q_{m}\frac{{bC}_{e}}{1 + {bC}_{e}}}} & (1)\end{matrix}$where q (mg/g) is the amount of the cation sorbed at the equilibriumconcentration C_(e) (ppm), q_(m) is the maximum sorption capacity of thesorbent and b (L/mg) is the Langmuir constant related to the free energyof the sorption.

The fitting results indicated a maximum sorption capacity of 402±14 mgCr₂O₇ ²⁻/g, which was the highest sorption capacity reported so far forMOFs (Table 2) and, in general, anion exchange sorbents (LDHs, organicresins and porous organic polymers show dichromate sorption capacitiesin the range 90-358 mg/g).

This sorption capacity corresponded to the insertion of 4.5 Cr₂O₇ ²⁻ions per formula unit of MOR-2. The affinity of the sorbent for Cr₂O₇ ²⁻can be expressed by the distribution coefficient (K_(d)) defined by theequation (eq. 2)

$\begin{matrix}{K_{d} = \frac{V\left\lbrack {\left( {C_{0} - C_{e}} \right)/C_{e}} \right\rbrack}{m}} & (2)\end{matrix}$where C₀ and C_(e) are the initial and equilibrium concentration ofCr₂O₇ ²⁻ (ppm), respectively; V is the volume (ml) of the testingsolution; and m is the amount of the ion exchanger (g) used in theexperiment (Manos, M. J.; Kanatzidis, M. G. Chem. Sci. 2016, 7, 4804).

The K_(d) values, calculated for a relatively wide range of initialconcentrations (21.6-216 ppm), were found to be 1.2×10⁴-1.19×10⁵ mL/g,which is particularly high (K_(d) values above 10⁴ mL/g are consideredexcellent).

Regeneration of MOR-2 after the Cr₂O₇ ²⁻ sorption could be achieved bytreatment of the Cr(VI)-loaded material with 4M HCl acid. Detailedstudies of regeneration/reuse of the sorbent are reported below in thesection for the column experiments.

The isotherm dichromate exchange data for the MOR-2-HA material was alsodetermined. The fitting of the results with the Langmuir model revealeda maximum sorption capacity of 338±19 mg/g. Although this value wassmaller than that for MOR-2, it was still higher than the sorptioncapacities of reported MOF-based sorbents (Table 2).

TABLE 2 Selected Cr(VI) sorption characteristics of reported MOFsKinetic Sorption studies- capacity Equilibrium MOF (mg/g) time at RTReference CrO₄ ²⁻ 1-ClO₄ 62.9 6 h P. F. Shi, B. Zhao, G. Xiong, Y. L.Hou and P. Cheng, Chem. Commun., 2012, 48, 8231 SLUG-21 60 48 h H. H.Fei, M. R. Bresler and S. R. J. Oliver, J. Am. Chem. Soc., 2011, 133,11110. Zn—Co-SLUG-35 68.5 2 h H. H. Fei, C. S. Han, J. C. Robins, and S.R. J. Oliver, Chem. Mater. 2013, 25, 647. MOR-2 264 1 min This workMOR-2-HA 243 1 min This work Cr₂O₇ ²⁻ ABT•2ClO₄ 213-271 48 h X. X. Li,H. Y. Xu, F. Z. Kong and R. H. Wang, Angew. Chem. Int. Ed., 2013, 52,13769. FIR-53 74 10 min H. R. Fu, Z. X. Xu and J. Zhang, Chem. Mater.2015, 27, 205. FIR-54 103 30 min H. R. Fu, Z. X. Xu and J. Zhang, Chem.Mater. 2015, 27, 205. ZJU-101 245 10 min Q. Zhang, J. Yu, J. Cai, L.Zhang, Y. Cui, Y. Yang, B. Chen and G. Qian, Chem. Commun., 2015, 51,14732. MOF-867 53 >12 h Q. Zhang, J. Yu, J. Cai, L. Zhang, Y. Cui, Y.Yang, B. Chen and G. Qian, Chem. Commun., 2015, 51, 14732. MOR-1-HA242-280 3-9 min S. Rapti, A. Pournara, D. Sarma, I. T. Papadas, G. S.Armatas, A. C. Tsipis, T. Lazarides, M. G. Kanatzidis and M. J. Manos,Chem. Sci. 2016, 7, 2467; and Y. S. Hassan, M. H. Alkordi, M. G.Kanatzidis and M. J. Manos, Inorg. Chem. Front. 2016, 3, 635. 1-SO₄ 16672 h A. V. Desai, B. Manna, A. Karmakar, A. Sahu and S. K. Ghosh, Angew.Chem. Int. Ed. 2016, 55, 7811. MOR-2 402 1 min This work MOR-2-HA 338 1min This workKinetics

Interestingly, the sorption of dichromate anions by MOR-2 was found tobe exceptionally fast. Equilibrium was reached within 1 min (for aninitial Cr₂O₇ ²⁻ concentration of 21.6 ppm) and 99.1% removal capacitywas observed (FIG. 25). This was the fastest Cr(VI) sorption rateobserved for MOFs (Table 2) and other anion-exchange sorbents (e.g.,LDHs require several hours to reach the ion exchange equilibrium). Thisresult reflected the rapid diffusion of Cr₂O₇ ²⁻ ions into thecrystalline porous network of MOR-2 and the particularly strong bindingof dichromate by the pyridinium-methyl-ammonium functional groups of thematerial. The sorption kinetics were also studied for the MOR-2-HAcomposite. The removal of dichromate could be completed within 1 min, asin the case of pristine MOR-2 material. Presumably, the small (1% wt.)content of alginic acid had negligible influence on the dichromateexchange kinetics of the composite.

pH Dependence of Sorption

The sorption of dichromate was also investigated in solutions of variouspH values (in the range 1-8). The results revealed 98.7-99.6% removal inpH˜2-8 (initial dichromate concentration was 21.6 ppm) and highlyefficient removal capacity (˜87.8%) even at pH˜1.

Selectivity Experiments

Common competitive ions for Cr(VI) ion exchange include Cl⁻, NO₃ ⁻, Br⁻and SO₄ ²⁻. Dichromate exchange experiments in the presence of Cl⁻, NO₃⁻ or Br⁻ indicated almost no effect on the Cr(VI) anion exchangeprocess, since very high dichromate removal capacities (93-94%) wereobtained even with 1000-fold excess of the competitive anions. Thisselectivity of MOR-2 for Cr₂O₇ ²⁻ was not only due to the higher chargeof this anionic species (compared to Cl⁻, NO₃ ⁻ or Br⁻), but also to itsstrong interactions with the functional groups of the material (seebelow). SO₄ ²⁻ had a larger effect on the dichromate sorption capacity.Still, high removal capacities (52-96%) could be obtained with 2-4-foldexcess of SO₄ ²⁻. It should be noted that in industrial chrome platingsolutions, the weight content of SO₄ ²⁻ is much lower (80-100 times)than that of Cr(VI) species (N. V. Mandich and D. L. Snyder,Electrodeposition of Chromium, Modern Electroplating, 5^(th) Edition,pg. 205-249; V. Boddu, K. H Abburi, J. L. Talbott and E. Smith, Environ.Sci. Technol., 2003, 37, 4449; L.-Y. Chang, Chrome reduction and heavymetals removal from wastewater—A pollution prevention approach,Proceedings of WM-01 Conference, Feb. 25-Mar. 1, 2001, Tucson, Ariz.).In such a concentration, the SO₄ ²⁻ is not a serious competitor fordichromate exchange by MOR-2, as will be shown below in the experimentswith industrial wastewater samples.

Chromate Ion Exchange

The sorption of chromate species by MOR-2 in terms of isothermdetermination and kinetic studies were also investigated. Previous tothis study no MOF had been investigated for sorption of both dichromateand chromate ions (see Table 2).

The isotherm exchange data were obtained at pH˜7 (FIG. 24B). Fitting wasperformed with the Langmuir model, and the results indicated a maximumcapacity of 264±10 mg/g. This value indicated exchange of 8 Cl⁻ by 6Cr(VI) ionic species (4 HCrO⁴⁻ and 2 CrO₄ ²⁻, see below). This capacitywas ˜4 times higher than that of MOF-based chromate exchangers (Table2). A Pb²⁺ MOF recently reported showed higher chromate capacity (˜324mg/g) than MOR-2. However, a Pb²⁺ material would be of no practicalinterest for environmental remediation (L. Aboutorabi, A. Morsali, E.Tahmasebi, O. Buyukgungor, Inorg. Chem. 2016, 55, 5507). In addition,exceptional high K_(d) values (up to 2.2×10⁵ mL/g) were obtained for thechromate exchange by MOR-2.

The investigation of the kinetics of CrO₄ ²⁻ exchange revealed that thesorption is completed within 1 min (FIG. 25), and ≥99.2% removal wasobserved. This excellent sorption rate, the fastest observed among MOFs(Table 2) and other materials, indicated strong interactions betweenCrO₄ ²⁻ and the functional groups of MOR-2.

The selectivity of MOR-2 for chromate vs. various competitive ions (Cl⁻,NO₃ ⁻, CO₃ ²⁻ etc) was very high, as observed from the results withpotable and industrial water samples described below.

The isotherm chromate sorption and kinetics data were also obtained forthe composite MOR-2-HA. The results revealed that the maximum sorptioncapacity of the composite (243±15 mg/g) was very close to that of MOR-2.In addition, the chromate sorption rate for MOR-2-HA was as fast as thatfor MOR-2, with the exchange equilibrium reached within 1 min (≥99.5%removal capacity was observed).

Experiments with Potable and Industrial Water Solutions

Although a number of MOF materials have been studied for their Cr(VI)exchange properties, there was a lack of data for sorption tests withreal-world samples. Thus, the Cr(VI) sorption properties of MOR-2 forindustrial wastewater samples and potable water solutions intentionallycontaminated with traces of Cr(VI) were investigated. The results arepresented in Table 3.

The industrial wastewater used in the ion exchange investigations waschrome plating water samples. The chrome plating wastewater wasgenerated by rinsing the plated parts upon their removal from theplating bath. Depending on the amount of water used in the rinsing step,the Cr(VI) concentrations ranged from very high (>1000 ppm) to moderate(<100 ppm) or low (<10 ppm) levels. In addition, the chrome platingwastewater included a small amount of H₂SO₄ (typically the mass ratio ofCrO₃ to H₂SO₄, used for the preparation of the chrome plating solution,is 50-100).

Two different types of chrome plating waste were provided by a metalplating company (located in Thessaloniki, Greece): One acidic sample (A)with dichromate ions in extremely high concentration (5170 ppm, pH˜1.8);and a second alkaline one (B) with chromate ions in moderateconcentration (52 ppm, pH˜8).

Sample A was too concentrated (˜5170 ppm) to be treated on a laboratoryscale. Thus, Cr(VI)-containing wastewater was prepared by diluting theoriginal sample A to ˜1 ppm of Cr20₇ ²⁻ (pH˜3 after the dilution), andsubsequently used in ion-exchange tests with MOR-2. Both original anddiluted (˜1 ppm CrO₄ ²⁻) samples B were treated by MOR-2.

The results of the ion-exchange experiments with the chrome platingsolutions indicated removal capacities in the range 90-96.% (Table 3).For the experiments with the dilute samples, the total Cr content in thefinal solutions was found <50 ppb, below the acceptable limit in water(100 and 50 ppb for the US EPA and EU, respectively).

The potable water samples were natural spring water solutions in whichCrO₄ ²⁻ traces (total Cr was 31-448 ppb) were added. These samplescontained a huge excess of various competitive anions, such as Cl⁻, HCO₃⁻ and SO₄ ²⁻. Specifically, the molar concentrations of Cl⁻, HCO₃ ⁻ andSO₄ ²⁻ were 68-978, 167-2391 and 34-483 times higher respectively thanthat of Cr(VI). Still, MOR-2 showed a remarkable capability to captureCrO₄ ²⁻ from these solutions and the removal capacities were found to be90-99.6%. The final total Cr content of the potable water solutions,after their treatment with MOR-2, was found to be 2-9 ppb, i.e., wellbelow the acceptable Cr levels (Table 3). These results revealed anexceptional selectivity of MOR-2 for CrO₄ ²⁻ anions.

TABLE 3 Results of ion exchange experiments with industrial and potablewater solutions Sample pH C₀(ppb)^(a) C_(e)(ppb)^(a) % removal Chromeplating^(b) 8 23300 870^(c)   96.3 Chrome plating^(d) 7 448.3 42.2  90.6Chrome plating^(e) 3 481.5 49.8  89.7 Potable water^(f) 7 31.4 3.1 90.1Potable water^(f) 7 107.6 9.0 91.6 Potable water^(f) 7 273.5 2.0 99.3Potable water^(f) 7 448.3 1.8 99.6 ^(a)Total Cr measured with ICP-MS;^(b)As received sample; ^(c)This final concentration was achieved byusing V/m ratio of 500 mL/g. In all other experiments, V/m ratio of 1000mL/g was used; ^(d)Sample after dilution of the chrome plating wastewith initial Cr concentration of 23.3 ppm, pH~8; ^(e)Sample afterdilution of the chrome plating waste with initial Cr concentration of5170 ppm, pH~2; ^(f)Natural spring water with Ca²⁺: 30.5 ppm, Mg²⁺: 12.2ppm, K⁺: 1.2 ppm, Na⁺: 21.4 ppm, HCO₃ ⁻: 88 ppm, Cl⁻: 21 ppm, SO₄ ²⁻: 28ppm.Column Ion-Exchange DataInitial Check of the Sorbents

As reported above, MOR-2 is not suitable for use in ion-exchange columnssince it forms very fine suspensions in water. Thus, passing waterthrough a column containing MOR-2 mixed with silica sand (an inertmaterial) resulted in the formation of a fine water suspension flowingout of the column. In contrast, MOR-2-HA remained fixed in the columnand the effluents were clear solutions as indicated by testing them witha laser beam.

The stationary phases in the columns used for the ion exchangeexperiments were mixtures of MOR-2-HA and silica sand. The use of suchmixtures instead of pure MOR-2-HA provided a stable flow of the solutionthrough the column due to the immobilization of the composite particlesin the sand. Furthermore, mixing the composite with an abundant materialsuch as sand was economically attractive. In fact, the columns preparedcontained only 1-2 wt. % of MOR-2-HA, so the main component of thestationary phase was sand. Still, the columns were highly efficient forthe decontamination of Cr(VI)-containing solutions.

Dichromate Ion Exchange

Column exchange experiments with a solution of dichromate with aninitial concentration of 108 ppm and a stationary phase containing 5 and0.05 g of sand and MOR-2-HA, respectively, were carried out. Thedichromate sorption could be seen even with the naked eye. The resultsindicated that 11 bed volumes (bed volume=bed height (cm)×crosssectional area (cm²)=3.5 mL) of the effluent samples showed a total Crcontent ≤10 ppb, i.e., well below the acceptable safety levels (FIG.26A). The column could be easily regenerated by treating it with 4 M HClacid. The regeneration process could be visually observed by thedecolorization of the (yellow-brown colored) column. A second run showedonly a small decrease of the sorption capacity (1 bed volume less)compared with the initial run of the column. Even after 5 runs of thecolumn, the sorption capacity remained high (9 bed volumes till thebreakthrough) (FIG. 26A). The breakthrough capacity Q_(b) (mg) could bedefined by the equation (eq. 3)Q=C _(o) ×V _(b)  (3)where C_(o) and V_(b) are the initial concentration of dichromate (mg/L)and the volume (L) of the effluent passing until the breakpointconcentration (defined as the maximum allowed level of the contaminant).

Thus, the breakthrough capacities from the five runs of the column were˜4.2 (1^(st) run), 3.8 (2^(nd) and 3^(rd) runs) and 3.4 (4^(th) and5^(th) runs) mg (FIG. 26A, inset).

In order to calculate the total column sorption capacity, the column wasloaded until saturation was reached (the point that the concentration ofdichromate in the effluent was identical to the initial dichromateconcentration) (FIG. 26B). Then, the data were fitted with the Thomasequation (eq. 4):

$\begin{matrix}{\frac{C}{C_{0}} = \frac{1}{1 + {\exp\left( {\frac{k_{Th}}{Q}\left( {{q_{\max}m} - {C_{0}V_{eff}}} \right)} \right)}}} & (4)\end{matrix}$where C, C₀ are the concentration (mg/L) of the ion in the effluent andits initial concentration (mg/L), respectively; k_(Th)(L mg⁻¹ min⁻¹) isthe Thomas model or sorption rate constant; q_(max)(mg/g) is thepredicted maximum sorption capacity; m(mg) is the mass of the sorbent; Q(mL min⁻¹) is the volumetric flow and V_(eff) is the effluent volume(mL).

This model assumes that the external (fluid-film) and the intra-particlemass transfer resistance have no effect on the column ion exchangeprocess.

The results of the fitting revealed a maximum sorption capacity of ˜100mg/g, close to the experimentally calculated (98.2 mg/g). The latter wasdetermined by the difference of the dichromate content of the initialand effluent solutions. Another important parameter for an ion-exchangecolumn is the degree of column utilization, which is defined as theratio of breakthrough to total column sorption capacity. This ratioshould be as close as possible to 1 (or 100%). The degree of columnutilization in the case of the MOR-2-HA/sand column was calculated to be84.6%. Such a high value indicated highly efficient performance of thecolumn.

Column experiments after doubling the quantity (100 mg) of MOR-2-HA inthe column were also performed. Thus, for the same initial dichromateconcentration (108 ppm), more than double breakthrough capacity (9.8 mg)was obtained. Even after 5 runs of the column, the breakthrough capacityremained relatively high (6.8 mg).

Column ion exchange experiments also have been conducted with adichromate solution of very low concentration (total Cr content was 0.48ppm, pH˜3) which, however, was above acceptable levels. The treatment ofsuch dilute solutions is usually challenging, since conventional methodslike precipitation are not effective in removing the contaminants in ppblevels. Remarkably, a MOR-2-HA/sand column containing only 50 mg of thecomposite (and 5 g of sand) was found to be efficient for thedecontamination of 3 L (˜857 bed volumes) of the dilute Cr(VI) solution,which showed a total Cr concentration <12 ppb after passing it throughthe column (FIG. 27).

Chromate Ion Exchange

Chromate ion exchange with a MOR-2-HA/sand column was also investigated(the stationary phase was composed of 0.1 and 5 g of MOR-2-HA and sand,respectively). This was the first study conducted for a MOF-basedsorbent. Five column ion exchange runs were performed (initial CrO₄ ²⁻concentration was 52 ppm, pH˜7). Upon sorption of chromate anions, thestationary phase of the column turned yellow. The regeneration of thesorbent was carried out by acid treatment (HCl 4M), and the stationaryphase restored its initial color.

In the first and second column runs, 11 bed volumes (breakthroughcapacity=2 mg of CrO₄ ²⁻) of the effluent solution contained total Cr ina concentration <47 ppb (i.e. below the acceptable limit) (FIG. 28). Asmall decrease (two bed volumes less) of the breakthrough sorptioncapacity (=1.638 mg of CrO₄ ²⁻) was observed for the third run (FIG.28). However, this breakthrough capacity remained unchanged even after a5^(th) run of the column (FIG. 28).

Column Tests with Industrial (Chrome-Plating) Samples

The last part of the column ion exchange studies involved tests withchrome plating wastewater samples. One sample tested was the diluted(total Cr˜0.48 ppm, pH˜3) chrome plating solution A (see above). TheMOR-2-HA/sand column was found particularly capable of decontaminatingthis wastewater sample. Specifically, 2.5 L (˜714 bed volumes) of thediluted chrome plating solution had a total Cr content <18 ppb, afterpassing it through the MOR-2-HA/sand column (FIG. 29).

This result was similar to that obtained with the laboratory prepareddichromate solution (FIG. 27).

The second industrial sample tested was the original (as received)chrome plating solution B (initial total Cr content ˜23.3 ppm, pH˜8).The MOR-2-HA/sand column was to be found highly efficient for thedecontamination of this wastewater sample. This process could even bevisually observed. The color of the stationary phase changed to yellowafter it was fully loaded by the Cr(VI) species. The regeneration of thecolumn also could be achieved by its treatment with 4 M HCl solution,and the stationary phase restored its original color.

Five ion exchange runs were carried out. In the first run, 7 bed volumesof the effluent solution had a total Cr content <10 ppb (Cr removal˜100%, breakthrough capacity ˜1.27 mg) (FIG. 30A). A small decrease inthe breakthrough capacity (only 1 bed volume less, or 1.09 mg) wasobserved in the second and third runs (FIG. 30A). Even in the 4^(th) and5^(th) runs, the column largely retained its initial breakthroughcapacity (5 bed volumes, 0.91 mg) (FIG. 30A).

The column sorption data for the five runs of the column could be welldescribed by the Thomas equation. The total column sorption capacitiespredicted by the Thomas model were 11-14.6 mg CrO₄ ²⁻/g of MOR-2-HA.Those are close to the experimentally found capacities (10.1-13.8 mg/g)(FIG. 30B). In addition, a high degree of column utilization(83.4-92.6%) was found for the column ion exchange tests with the chromeplating solution B, which is another indication of the excellentperformance of the MOR-2-HA/sand column.

Mechanism of the Cr(VI) Sorption Processes

In aqueous solution, hexavalent chromium exists as oxido-forms in avariety of species depending on pH and Cr(VI) concentration. For the oxospecies of hexavalent chromium, three main pH regions may bedistinguished:H₂CrO₄ (pH<0);  (1)HCrO₄ ⁻ and Cr₂O₇ ²⁻ (pH 2-6); and  (2)CrO₄ ²⁻ (pH>6).  (3)

Depending on the concentration and acidity, hexavalent chromium canexist either as chromate CrO₄ ²⁻ or dichromate Cr₂O₇ ²⁻. The commondissolved chromium species (all hexavalent chromium) are HCrO₄, CrO₄ ²⁻and Cr₂O₇ ²⁻. Another species possibly present in aqueous solutions ofoxido-Cr(VI) species is chromic anhydride CrO₃, which can be formed inthe course of the strongly exothermic interactions of CrO₄ ²⁻ or Cr₂O₇²⁻ anions with protons from the strongly acidic aqueous solutions, orwith the protons of the protonated amine groups of MOR-2, according tothe reactions (eq. 5, 6):CrO₄ ²⁻+2H⁺→CrO₃+H₂O+334.4 kcal/mol  (5)Cr₂O₇ ²⁻+2H⁺→2CrO₃+H₂O+258.1 kcal/mol  (6)

Which entity will dominate in a particular environment depends upon thespecific conditions, including, for example, pH, E_(h) (redoxpotential), total concentration of chromium, and the overall aqueouschemistry.

Therefore, the possible interaction modes of MOR-2@CrO₄ ²⁻, MOR-2@HCrO₄,MOR-2@Cr₂O₇ ²⁻ and MOR-2@CrO₃, and the thermodynamics of possiblereactions involved in the oxido-Cr(VI) anion exchange processes takingplace in the MOR-2-HA columns by means of DFT computational protocols,were explored. Comparable results were obtained employing thewB97XD/Def2-TZVPP DFT method.

The proton affinities of pyridine and methylamine moieties of thePhNHCH₂Py ligand used as a model of the PATP ligand predicted to be−157.5 and −151.7 kcal/mol, respectively, at the BP86/6-31G(d,p) levelof theory indicate the slightly more basic character of pyridinecompared to methylamine moieties. On the other hand, the protonaffinities of CrO₄ ²⁻ and Cr₂O₇ ²⁻ are predicted to be −203.0 and −164.5kcal/mol, respectively. The detachment of the proton either from thepyridinium or methylammonium moieties of the [PhNH₂CH₂PyH]²⁺ ligand bythe CrO₄ ²⁻ dianions according to the reactions (eq. 7, 8):[PhNH₂CH₂PyH]²⁺+CrO₄ ²⁻→[PhNH₂CH₂Py]⁺+HCrO⁴⁻  (7)[PhNH₂CH₂PyH]²⁺+CrO₄ ²⁻→[PhNHCH₂PyH]⁺+HCrO₄ ⁻  (8)are slightly less exothermic (−45.1 kcal/mol) for the proton detachmentprocess from pyridinium than methylammonium moieties (−51.3 kcal/mol).The deprotonation of the pyridinium or methylammonium moieties by thechromate anions was clearly shown in the structures of the[PhNH₂CH₂PyH(CrO₄)] and [PhNH₂(CrO₄)CH₂PyH] associations optimized atthe BP86/6-31G(d,p) level of theory (FIG. 31).

It could be observed that the CrO₄ ²⁻ anions deprotonating either thepyridinium or methylammonium moieties were transformed to HCrO₄ ⁻species associated with the [PhNHCH₂Py] ligand through hydrogen bonds.

The HCrO₄ ⁻ species interacting with the [PhNH₂CH₂Py]⁺ andPhNH₂CH₂PyH]²⁺ ligands yielded the [PhNH₂(HOCrO₃)CH₂Py] andPhNH₂CH₂PyH(HOCrO₃)]⁺ weak associations, respectively, supported by N—H. . . O—H hydrogen bonds (FIG. 31). The interaction energies for the[PhNH₂(HOCrO₃)CH₂Py] and PhNH₂CH₂PyH(HOCrO₃)]⁺ associations are 11.6 and23.9 kcal·mol respectively at the BP86/6-31G(d,p) level of theory.Similarly, the deprotonation of the pyridinium or methylammoniummoieties of the [PhNH₂CH₂PyH]²⁺ ligand by the Cr₂O₇ ²⁻ dianionsaccording to the reactions:[PhNH₂CH₂PyH]²⁺+Cr₂O₇ ²⁻→[PhNH₂CH₂Py]⁺+HCr₂O₇ ⁻[PhNH₂CH₂PyH]²⁺+Cr₂O₇ ²⁻→[PhNHCH₂PyH]⁺+HCr₂O₇ ⁻are predicted to be slightly exothermic, the estimated exothermicitiesbeing −7.1 and −12.8 kcal/mol, respectively. Therefore, the dichromateanions do not deprotonate the pyridinium or methylammonium moieties ofthe [PhNH₂CH₂PyH]²⁺ ligand. Rather, interacting with [PhNH₂CH₂PyH]²⁺ligand yields the weak association [PhNH₂CH₂PyH(Cr₂O₇)] supported bythree hydrogen bonds (FIG. 31). The estimated interaction energy was−37.3 kcal/mol.

The above calculations indicated that (no protonated) the dichromateanions were capable of strongly interacting with the functional groupsof MOR-2. The results from the sorption experiments revealed that 4.5Cr₂O₇ ²⁻ anions (per formula unit of the material) could be insertedinto the pores of the material at the same time all Cl⁻ anions wereremoved. Combining the theoretical and experimental findings, it issuggested that 4Cr₂O⁷ ²⁻ exchange 8 Cl⁻ anions. Additional 0.5 Cr₂O₇ ²⁻could be incorporated into the structure, presumably with thesimultaneous removal of one OH⁻ terminal ligand (which may be replacedby a water molecule).

In the case of chromate exchange, the calculations clearly showed thetendency of CrO₄ ²⁻ to withdraw one proton from either pyridinium ormethylammonium functional groups of MOR-2, thus forming HCrO₄ ⁻ species.The chromate exchange data revealed that ˜6 Cr(VI) species are insertedper formula unit of the material, with the simultaneous removal of 8 Cl⁻anions. Given the capability of chromate anions to deprotonate thepyridinium or methylammonium moieties, it is suggested that 4 HCrO₄ ⁻will be incorporated into the pores of the material, removing 4 Cl⁻anions. The remaining 2 Cr(VI) species, exchanging the rest of Cl⁻anions, will be thus in the form of no protonated chromate anions.

All of the above can be summarized by the following equations (eq. 9,10):H₁₆[Zr₆O₁₆(H₂PATP)₄]Cl₈+4.5Cr₂O₇²⁻+H₂O→H₁₇[Zr₆O₆(H₂PATP)₄](Cr₂O₇)_(4.5)+8Cl—+OH⁻  (9)H₁₆[Zr₆O₁₆(H₂PATP)₄]Cl₈+6CrO₄²⁻→H₁₆[Zr₆O₁₆(HPATP)₄](HCrO₄)₄(CrO₄)₂+8Cl⁻  (10)

Importantly, among all oxido-Cr(VI) species participating in the anionexchange processes, only chromic anhydride, CrO₃, is attached to the Ndonor atoms of the HPATP and PATP model ligands yielding tetrahedral[PhNH₂CH₂Py(CrO₃)]⁺, [PhNHCH₂Py(CrO₃)], [PhNH(CrO₃)CH₂PyH]⁺,[PhNH(CrO₃)CH₂Py], and [PhNH(CrO₃)CH₂Py(CrO₃)] complexes (FIG. 31).

The estimated binding energies for the [PhNH₂CH₂Py(CrO₃)]⁺,[PhNHCH₂Py(CrO₃)], [PhNH(CrO₃)CH₂PyH]⁺, [PhNH(CrO₃)CH₂Py], and[PhNH(CrO₃)CH₂Py(CrO₃)] complexes were 34.0, 23.2, 39.6, 35.3 and 35.5kcal/mol, respectively. It should be noted that CrO₃ formed strongercoordination bonds when coordinated to pyridine N donor atom than toamine N donor atom. The stronger Pyr-CrO₃ bond, compared to the PhN—CrO₃bond, is reflected by the Pyr-CrO₃ and PhN—CrO₃ bond lengths, with theformer being shorter than the latter.

The regeneration of the MOR-2-HA columns by treating them withconcentrated HCl solutions (4 M) can be easily explained by the acidichydrolysis of the weak associations, and the complexes involved in theoxido-Cr(VI) anion exchange processes taking place in the columns.

Photophysical Properties and Luminescence Sensing

The photophysical properties of MOR-2 were studied by solid state UV-visdiffuse reflectance and steady state emission spectroscopy. The diffusereflectance spectrum of MOR-2 (FIG. 32) showed an absorption band in theUV region (λ_(max)=266 nm) and a lower energy absorption signal whichmaximized at 380 nm and tailed off in the visible region at ca. 450 nm.These bands were attributed to ligand based singlet π-π* and n-π*transitions with the latter involving the lone pair on the amino group.The high-energy band also included a contribution from Zr cluster-basedtransitions. In the exchanged materials MOR-2@CrO₄ ²⁻ and MOR-2@Cr₂O₇ ²⁻(vide supra), absorption extended further in the visible region tailingoff at ca. 580 and 720 nm, respectively (FIG. 32). In the case ofMOR-2@Cr₂O₇ ²⁻, a shoulder at approximately 605 nm was also observed.These additional absorption features reflected the presence of theoxido-to-metal-charge-transfer transitions (LMCT) of the chromiumspecies.

Upon excitation at 360 nm, MOR-2 exhibited turquoise fluorescence. Theemission spectrum of MOR-2 (FIG. 33) included a broad band with maximumat ca. 470 nm originating mainly from radiative deactivation of asinglet n-π* excited state. The fluorescence of MOR-2, in combinationwith its ability to rapidly and efficiently sorb Cr(VI) from water,prompted the testing of MOR-2 as a luminescent sensor for Cr(VI) inaqueous media.

A titration experiment in which aliquots of a 10⁻⁴ M aqueous standardstock solution of K₂Cr₂O₇ were added to a suspension of MOR-2 at pH 3led to strong fluorescence quenching (FIG. 33). The fluorescenceintensity continually decreased with concentrations of 25 ppm, 51 ppm,76 ppm, 101 ppm, 126 ppm, 151 ppm, 180 ppm, 210 ppm, 249 ppm, 297 ppm,345 ppm, 392 ppm, and 485 ppm. At the end of the titration, loss of morethan 80% of the initial emission signal was observed, which was alsoevident to the naked eye. Notably, the MOR-2 was activated overnight in4 M HCl prior to its use in order to ensure that pyridine and aminomoieties were fully protonated. At this pH and at the low concentrationsof the fluorescence experiments, Cr(VI) in solution was almost totallyin the form of HCrO₄ ⁻ ions. In the theoretical work of the previoussection it is shown that HCrO₄ ⁻ may interact via charge-assistedhydrogen bonding with the [ArNH₂CH₂PyH]²⁺ units of MOR-2. However, it ismore possible that once HCrO₄ ⁻ ions enter the pores of MOR-2, theequilibrium 2HCrO₄ ⁻⇄Cr₂O₇ ²⁻+H₂O shifts to the right for two reasons:i) the concentration of Cr(VI) within the pores is greater than that insolution; and ii) the dichromate species is able to interact morestrongly with the [ArNH₂CH₂PyH]²⁺ units via the formation of threecharge-assisted hydrogen bonds, thereby providing a driving force whichrenders its formation more favorable. A superposition of the absorptionspectrum of MOR-2@Cr₂O₇ ²⁻ and the emission spectrum of MOR-2 showedthat there was a clear overlap between the latter and the absorptionfeatures attributable to the dichromate ion. Therefore, the observedemission quenching in the titration experiment may have been a result ofenergy transfer from the excited n-π* levels of the aromatic bridgingligands to the dichromate LMCT transitions. Additionally, given thegreat oxidizing ability of dichromates and the electron-donating natureof amino terephthalate derivatives, it is also highly possible that thequenching mechanism involved a bridging ligand to Cr(VI) electrontransfer component.

Analysis of the calibration curve of the fluorescence titration allowedthe limits of detection (LOD) and quantification (LOQ) at 4 and 13 ppb,respectively, to be determined. These values demonstrated the extremelyhigh sensitivity of MOR-2 towards Cr(IV), as both the LOD and LOQ valueswere well below the EU and US EPA acceptable levels of Cr(VI) in water(50 and 100 ppb respectively).

To further assess the sensing ability of MOR-2 towards Cr(IV) in realsamples, two additional sensing experiments were performed using thelower concentration chrome plating waste sample B (vide supra) as astock solution, after adjusting its pH to 3 and diluting it to achieve aCr(VI) concentration of 10 ppm. In the first experiment, distilled waterwas used as a solvent both for the dilution of the Cr(VI) sample and asa suspension medium for MOR-2, while in the second experiment distilledwater was replaced with potable water containing 10.5 ppm of the maincompeting anion SO₄ ²⁻ (vide supra). As seen in FIG. 34A, in the firstexperiment, the system showed comparable response as in the case of thestandard Cr(VI) sample, giving LOD and LOQ values of 6 and 18 ppb,respectively. However, when potable water was used as solvent (FIG.34B), a considerable decrease in sensitivity, with LOD and LOQ values of35 and 110 ppb, respectively, was observed. These results are agreementwith the competition results described above, which showed that in thepresence of excess SO₄ ²⁻ ions, the selectivity of MOR 2 towardsdichromate ions was somewhat hindered. As shown in FIG. 34A, in thefirst experiment, the fluorescence intensity continually decreased withconcentrations of 29 ppm, 57 ppm, 85 ppm, 113 ppm, 141 ppm, 169 ppm, 196ppm, 224 ppm, 279 ppm, 333 ppm, 387 ppm, 440 ppm, 492 ppm, 545 ppm, and648 ppm. As shown in FIG. 34B, in the second experiment, thefluorescence intensity continually decreased with concentrations of 57ppm, 113 ppm, 169 ppm, 224 ppm, 333 ppm, 440 ppm, 545 ppm, and 645 ppm.Nevertheless, even in the latter case the LOD value was lower than theacceptable levels of Cr(VI) in water.

CONCLUSIONS

In conclusion, MOR-2, a microporous Zr⁴⁺ MOF withpyridinium-methyl-ammonium functional groups, was described. MOR-2 wassynthesized via direct solvothermal reaction of Zr⁴⁺ salt and apre-functionalized ligand, a strategy that ensured incorporation of thehighest possible number of functional groups. MOR-2 represented a uniquesorbent with capability to effectively capture both chromate anddichromate species. In fact, MOR-2 exhibited the highest capacity andthe fastest kinetics for Cr(VI) sorption among all known materials.Remarkably, MOR-2 could sorb selectively Cr(VI) not only from syntheticsolutions, but also from industrial waste and drinking water. Thecomposite of MOR-2 with alginic acid (MOR-2-HA) was also prepared.Extensive Cr(VI) sorption studies were carried out with an ion exchangecolumn filled mainly with silica sand and only a small quantity ofMOR-2-HA (1-2 wt %). Such a simple and relatively inexpensive ionexchange column would be capable of decontaminating a large variety ofCr(VI)-containing solutions, including industrial waste with either highor quite low Cr(VI) content. The column could easily be regenerated bytreating it with HCl solution and would be reusable for several cycles,a significant aspect for applications in wastewater treatment.

Theoretical studies revealed that MOR-2, through its functional groups,was involved in relatively strong interactions with the Cr(VI) species,with the interaction energies ranging from −11.6 up to −61.3 kcal/mol.These results explained the excellent Cr(VI) sorption property of MOR-2.

Besides being an excellent sorbent, MOR-2 was also shown to be a highlyefficient sensor for Cr(VI) species, as shown by fluorescence titrationexperiments in acidic aqueous media. Maximum LOD and LOQ values were 4and 13 ppb, while the system showed excellent sensitivity whenreal-world, rather than standard, samples were used. Considerablesensitivity even in the presence of excess competing SO₄ ²⁻ anions wasobserved. Therefore, MOR-2 showed great promise as a fluorescent sensorfor Cr(VI) in water.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A composite material comprising: metal organicresin particles comprising metal organic frameworks and associatedcounter anions, wherein the metal organic frameworks comprise metalnodes coordinated via organic molecular linkers to form a connectedporous network and further wherein the organic molecular linkers areprotonated and amine-functionalized; and an organic polymer coating themetal organic resin particles.
 2. The composite material of claim 1,wherein the organic polymer is an alginic acid polymer.
 3. The compositematerial of claim 1, wherein the counter anions are halide anions. 4.The composite material of claim 3, wherein the halide anions arechloride ions.
 5. The composite material of claim 1, wherein the metalnodes of the metal organic frameworks are Zr₆ nodes.
 6. The compositematerial of claim 5, wherein the metal organic resins have the formula:[Zr₆O₄(OH)₈(H₂O)₄(H₂PATP)₄]X⁻ ₆, or the same formula, but with oxoligands, aquo ligands, or a combination thereof in place of some or allof the hydroxo ligands, where H₂PATP is2-((pyridine-1-ium-2-ylmethyl)ammonio)terephthalate and X is amonovalent anion.
 7. The composite material of claim 6, wherein theorganic polymer is an alginic acid polymer.
 8. The composite material ofclaim 5, wherein the metal organic resins have the formula:[Zr₆O₄(OH)₄(NH₃ ⁺-BDC)₆]X⁻ ₆, or the same formula, but with oxo ligands,aquo ligands, or a combination thereof in place of some or all of thehydroxo ligands, where BDC is 1,4-benzenedicarboxylate and X is amonovalent anion.
 9. The composite material of claim 8, wherein theorganic polymer is an alginic acid polymer.
 10. An anion exchange columncomprising: a column; and a mixture of an inert granular material andthe composite material of claim 1 packed within the column.
 11. Theanion exchange column of claim 10, wherein the mixture comprises nogreater than 3 wt. % of the composite material.
 12. The anion exchangecolumn of claim 10, wherein the inert granular material is sand.
 13. Amethod of removing metal anions from a sample, the method comprising:exposing a sample comprising metal anions to the composite material ofclaim 1, whereby the metal anions undergo anion exchange with thecounter anions of the composite material; and separating the compositematerial from the sample.
 14. A method of removing metal anions from asample using the anion exchange column of claim 10, the methodcomprising running a sample comprising metal anions through the anionexchange column, whereby the metal anions undergo anion exchange withthe counter anions of the composite material.
 15. The method of claim14, wherein the metal anions are chromium-containing anions,selenium-containing anions, or a combination thereof.
 16. A method ofmaking a composite material, the method comprising: heating a mixture ofa zirconium halide salt and NH₂-H₂BDC in an acidic aqueous solution,where NH₂-H₂BDC is 2-amino-terephthalic acid, whereby a reflux reactionbetween the zirconium halide salt and the NH₂-H₂BDC forms a suspensionof metal organic resin particles in the solution, the metal organicresin comprising zirconium nodes coordinated via organic molecularlinkers in a connected porous network, wherein the organic molecularlinkers are protonated and amine-functionalized; and adding an alkalimetal alginate salt to the suspension, whereby the alkali metal alginateconverts into alginic acid, which forms a water insoluble alginic acidpolymer coating on, and flocculates, the metal organic resin particles.17. A method of making a composite material, the method comprising:forming an aqueous solution comprising an alkali metal alginate salt andmetal organic resin particles, the metal organic resin particlescomprising metal nodes coordinated via organic molecular linkers in aconnected porous network, wherein the organic molecular linkers areprotonated and amine-functionalized, whereby one or more monolayers ofalginate-saturated water form a coating on the metal organic resinparticles; adding an alkali earth metal halide salt to the aqueoussolution, whereby a water-insoluble coating of an alkali earth metalalginate forms around the metal organic resin particles; removing thecoated metal organic resin particles from the aqueous solution; andreacting the coated metal organic resin particles with a hydrogen halideto protonate the amine-functionalized metal organic frameworks andconvert the alkali earth metal alginate coating into an alginic acidpolymer.
 18. A method of making a composite material, the methodcomprising: forming an aqueous solution comprising an alkali metalalginate salt and metal organic resin particles, the metal organic resinparticles comprising metal nodes coordinated via organic molecularlinkers in a connected porous network, wherein the organic molecularlinkers are protonated and amine-functionalized, whereby one or moremonolayers of alginate-saturated water form a coating on the metalorganic resin particles; and adding a hydrogen halide to the solution,whereby the hydrogen halide reacts with the alginate and the metalorganic resin particles to protonate the amine-functionalized metalorganic frameworks and to form an alginic acid polymer coating aroundthe organic resin particles.